/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *      https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  /*
   * This is not the original file distributed by the Apache Software Foundation
   * It has been modified by the Hipparchus project
   */
  package org.hipparchus.stat.inference;
  
  import org.hipparchus.distribution.continuous.TDistribution;
  import org.hipparchus.exception.LocalizedCoreFormats;
  import org.hipparchus.exception.MathIllegalArgumentException;
  import org.hipparchus.exception.MathIllegalStateException;
  import org.hipparchus.exception.NullArgumentException;
  import org.hipparchus.stat.LocalizedStatFormats;
  import org.hipparchus.stat.StatUtils;
  import org.hipparchus.stat.descriptive.StatisticalSummary;
  import org.hipparchus.util.FastMath;
  import org.hipparchus.util.MathUtils;
  
  /**
   * An implementation for Student's t-tests.
   * <p>
   * Tests can be:
   * <ul>
   * <li>One-sample or two-sample</li>
   * <li>One-sided or two-sided</li>
   * <li>Paired or unpaired (for two-sample tests)</li>
   * <li>Homoscedastic (equal variance assumption) or heteroscedastic
   * (for two sample tests)</li>
   * <li>Fixed significance level (boolean-valued) or returning p-values.</li>
   * </ul>
   * <p>
   * Test statistics are available for all tests.  Methods including "Test" in
   * in their names perform tests, all other methods return t-statistics.  Among
   * the "Test" methods, <code>double-</code>valued methods return p-values;
   * <code>boolean-</code>valued methods perform fixed significance level tests.
   * Significance levels are always specified as numbers between 0 and 0.5
   * (e.g. tests at the 95% level  use <code>alpha=0.05</code>).
   * <p>
   * Input to tests can be either <code>double[]</code> arrays or
   * {@link StatisticalSummary} instances.
   * <p>
   * Uses Hipparchus {@link org.hipparchus.distribution.continuous.TDistribution}
   * implementation to estimate exact p-values.
   */
  public class TTest { // NOPMD - this is not a Junit test class, PMD false positive here
  
      /** Empty constructor.
       * <p>
       * This constructor is not strictly necessary, but it prevents spurious
       * javadoc warnings with JDK 18 and later.
       * </p>
       * @since 3.0
       */
      public TTest() { // NOPMD - unnecessary constructor added intentionally to make javadoc happy
          // nothing to do
      }
  
      /**
       * Computes a paired, 2-sample t-statistic based on the data in the input
       * arrays.  The t-statistic returned is equivalent to what would be returned by
       * computing the one-sample t-statistic {@link #t(double, double[])}, with
       * <code>mu = 0</code> and the sample array consisting of the (signed)
       * differences between corresponding entries in <code>sample1</code> and
       * <code>sample2.</code>
       * <p>* <strong>Preconditions</strong>:</p>
       * <ul>
       * <li>The input arrays must have the same length and their common length
       * must be at least 2.
       * </li></ul>
       *
       * @param sample1 array of sample data values
       * @param sample2 array of sample data values
       * @return t statistic
       * @throws NullArgumentException if the arrays are <code>null</code>
       * @throws MathIllegalArgumentException if the arrays are empty
       * @throws MathIllegalArgumentException if the length of the arrays is not equal
       * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
       */
      public double pairedT(final double[] sample1, final double[] sample2)
          throws MathIllegalArgumentException, NullArgumentException {
  
          checkSampleData(sample1);
          checkSampleData(sample2);
         double meanDifference = StatUtils.meanDifference(sample1, sample2);
         return t(meanDifference, 0,
                  StatUtils.varianceDifference(sample1, sample2, meanDifference),
                  sample1.length);
     }
 
     /**
      * Returns the <i>observed significance level</i>, or
      * <i> p-value</i>, associated with a paired, two-sample, two-tailed t-test
      * based on the data in the input arrays.
      * <p>
      * The number returned is the smallest significance level
      * at which one can reject the null hypothesis that the mean of the paired
      * differences is 0 in favor of the two-sided alternative that the mean paired
      * difference is not equal to 0. For a one-sided test, divide the returned
      * value by 2.</p>
      * <p>
      * This test is equivalent to a one-sample t-test computed using
      * {@link #tTest(double, double[])} with <code>mu = 0</code> and the sample
      * array consisting of the signed differences between corresponding elements of
      * <code>sample1</code> and <code>sample2.</code></p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the p-value depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The input array lengths must be the same and their common length must
      * be at least 2.
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @return p-value for t-test
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the arrays are empty
      * @throws MathIllegalArgumentException if the length of the arrays is not equal
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public double pairedTTest(final double[] sample1, final double[] sample2)
         throws MathIllegalArgumentException, NullArgumentException, MathIllegalStateException {
 
         double meanDifference = StatUtils.meanDifference(sample1, sample2);
         return tTest(meanDifference, 0,
                 StatUtils.varianceDifference(sample1, sample2, meanDifference),
                 sample1.length);
     }
 
     /**
      * Performs a paired t-test evaluating the null hypothesis that the
      * mean of the paired differences between <code>sample1</code> and
      * <code>sample2</code> is 0 in favor of the two-sided alternative that the
      * mean paired difference is not equal to 0, with significance level
      * <code>alpha</code>.
      * <p>
      * Returns <code>true</code> iff the null hypothesis can be rejected with
      * confidence <code>1 - alpha</code>.  To perform a 1-sided test, use
      * <code>alpha * 2</code></p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The input array lengths must be the same and their common length
      * must be at least 2.
      * </li>
      * <li> <code> 0 &lt; alpha &lt; 0.5 </code>
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @param alpha significance level of the test
      * @return true if the null hypothesis can be rejected with
      * confidence 1 - alpha
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the arrays are empty
      * @throws MathIllegalArgumentException if the length of the arrays is not equal
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      * @throws MathIllegalArgumentException if <code>alpha</code> is not in the range (0, 0.5]
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public boolean pairedTTest(final double[] sample1, final double[] sample2,
                                final double alpha)
         throws MathIllegalArgumentException, NullArgumentException, MathIllegalStateException {
 
         checkSignificanceLevel(alpha);
         return pairedTTest(sample1, sample2) < alpha;
 
     }
 
     /**
      * Computes a <a href="http://www.itl.nist.gov/div898/handbook/prc/section2/prc22.htm#formula">
      * t statistic </a> given observed values and a comparison constant.
      * <p>
      * This statistic can be used to perform a one sample t-test for the mean.
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array length must be at least 2.
      * </li></ul>
      *
      * @param mu comparison constant
      * @param observed array of values
      * @return t statistic
      * @throws NullArgumentException if <code>observed</code> is <code>null</code>
      * @throws MathIllegalArgumentException if the length of <code>observed</code> is &lt; 2
      */
     public double t(final double mu, final double[] observed)
         throws MathIllegalArgumentException, NullArgumentException {
 
         checkSampleData(observed);
         // No try-catch or advertised exception because args have just been checked
         return t(StatUtils.mean(observed), mu, StatUtils.variance(observed),
                  observed.length);
     }
 
     /**
      * Computes a <a href="http://www.itl.nist.gov/div898/handbook/prc/section2/prc22.htm#formula">
      * t statistic </a> to use in comparing the mean of the dataset described by
      * <code>sampleStats</code> to <code>mu</code>.
      * <p>
      * This statistic can be used to perform a one sample t-test for the mean.
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li><code>observed.getN() &ge; 2</code>.
      * </li></ul>
      *
      * @param mu comparison constant
      * @param sampleStats DescriptiveStatistics holding sample summary statitstics
      * @return t statistic
      * @throws NullArgumentException if <code>sampleStats</code> is <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      */
     public double t(final double mu, final StatisticalSummary sampleStats)
         throws MathIllegalArgumentException, NullArgumentException {
 
         checkSampleData(sampleStats);
         return t(sampleStats.getMean(), mu, sampleStats.getVariance(),
                  sampleStats.getN());
     }
 
     /**
      * Computes a 2-sample t statistic,  under the hypothesis of equal
      * subpopulation variances.  To compute a t-statistic without the
      * equal variances hypothesis, use {@link #t(double[], double[])}.
      * <p>
      * This statistic can be used to perform a (homoscedastic) two-sample
      * t-test to compare sample means.</p>
      * <p>
      * The t-statistic is</p>
      * <p>
      * &nbsp;&nbsp;<code>  t = (m1 - m2) / (sqrt(1/n1 +1/n2) sqrt(var))</code>
      * </p><p>
      * where <strong><code>n1</code></strong> is the size of first sample;
      * <strong><code> n2</code></strong> is the size of second sample;
      * <strong><code> m1</code></strong> is the mean of first sample;
      * <strong><code> m2</code></strong> is the mean of second sample
      * and <strong><code>var</code></strong> is the pooled variance estimate:
      * </p><p>
      * <code>var = sqrt(((n1 - 1)var1 + (n2 - 1)var2) / ((n1-1) + (n2-1)))</code>
      * </p><p>
      * with <strong><code>var1</code></strong> the variance of the first sample and
      * <strong><code>var2</code></strong> the variance of the second sample.
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array lengths must both be at least 2.
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @return t statistic
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      */
     public double homoscedasticT(final double[] sample1, final double[] sample2)
         throws MathIllegalArgumentException, NullArgumentException {
 
         checkSampleData(sample1);
         checkSampleData(sample2);
         // No try-catch or advertised exception because args have just been checked
         return homoscedasticT(StatUtils.mean(sample1), StatUtils.mean(sample2),
                               StatUtils.variance(sample1), StatUtils.variance(sample2),
                               sample1.length, sample2.length);
     }
 
     /**
      * Computes a 2-sample t statistic, without the hypothesis of equal
      * subpopulation variances.  To compute a t-statistic assuming equal
      * variances, use {@link #homoscedasticT(double[], double[])}.
      * <p>
      * This statistic can be used to perform a two-sample t-test to compare
      * sample means.</p>
      * <p>
      * The t-statistic is</p>
      * <p>
      * &nbsp;&nbsp; <code>  t = (m1 - m2) / sqrt(var1/n1 + var2/n2)</code>
      * </p><p>
      *  where <strong><code>n1</code></strong> is the size of the first sample
      * <strong><code> n2</code></strong> is the size of the second sample;
      * <strong><code> m1</code></strong> is the mean of the first sample;
      * <strong><code> m2</code></strong> is the mean of the second sample;
      * <strong><code> var1</code></strong> is the variance of the first sample;
      * <strong><code> var2</code></strong> is the variance of the second sample;
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array lengths must both be at least 2.
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @return t statistic
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      */
     public double t(final double[] sample1, final double[] sample2)
         throws MathIllegalArgumentException, NullArgumentException {
 
         checkSampleData(sample1);
         checkSampleData(sample2);
         // No try-catch or advertised exception because args have just been checked
         return t(StatUtils.mean(sample1), StatUtils.mean(sample2),
                  StatUtils.variance(sample1), StatUtils.variance(sample2),
                  sample1.length, sample2.length);
     }
 
     /**
      * Computes a 2-sample t statistic, comparing the means of the datasets
      * described by two {@link StatisticalSummary} instances, without the
      * assumption of equal subpopulation variances.  Use
      * {@link #homoscedasticT(StatisticalSummary, StatisticalSummary)} to
      * compute a t-statistic under the equal variances assumption.
      * <p>
      * This statistic can be used to perform a two-sample t-test to compare
      * sample means.</p>
      * <p>
       * The returned  t-statistic is</p>
      * <p>
      * &nbsp;&nbsp; <code>  t = (m1 - m2) / sqrt(var1/n1 + var2/n2)</code>
      * </p><p>
      * where <strong><code>n1</code></strong> is the size of the first sample;
      * <strong><code> n2</code></strong> is the size of the second sample;
      * <strong><code> m1</code></strong> is the mean of the first sample;
      * <strong><code> m2</code></strong> is the mean of the second sample
      * <strong><code> var1</code></strong> is the variance of the first sample;
      * <strong><code> var2</code></strong> is the variance of the second sample
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The datasets described by the two Univariates must each contain
      * at least 2 observations.
      * </li></ul>
      *
      * @param sampleStats1 StatisticalSummary describing data from the first sample
      * @param sampleStats2 StatisticalSummary describing data from the second sample
      * @return t statistic
      * @throws NullArgumentException if the sample statistics are <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      */
     public double t(final StatisticalSummary sampleStats1,
                     final StatisticalSummary sampleStats2)
         throws MathIllegalArgumentException, NullArgumentException {
 
         checkSampleData(sampleStats1);
         checkSampleData(sampleStats2);
         return t(sampleStats1.getMean(), sampleStats2.getMean(),
                  sampleStats1.getVariance(), sampleStats2.getVariance(),
                  sampleStats1.getN(), sampleStats2.getN());
     }
 
     /**
      * Computes a 2-sample t statistic, comparing the means of the datasets
      * described by two {@link StatisticalSummary} instances, under the
      * assumption of equal subpopulation variances.  To compute a t-statistic
      * without the equal variances assumption, use
      * {@link #t(StatisticalSummary, StatisticalSummary)}.
      * <p>
      * This statistic can be used to perform a (homoscedastic) two-sample
      * t-test to compare sample means.</p>
      * <p>
      * The t-statistic returned is</p>
      * <p>
      * &nbsp;&nbsp;<code>  t = (m1 - m2) / (sqrt(1/n1 +1/n2) sqrt(var))</code>
      * </p><p>
      * where <strong><code>n1</code></strong> is the size of first sample;
      * <strong><code> n2</code></strong> is the size of second sample;
      * <strong><code> m1</code></strong> is the mean of first sample;
      * <strong><code> m2</code></strong> is the mean of second sample
      * and <strong><code>var</code></strong> is the pooled variance estimate:
      * </p><p>
      * <code>var = sqrt(((n1 - 1)var1 + (n2 - 1)var2) / ((n1-1) + (n2-1)))</code>
      * </p><p>
      * with <strong><code>var1</code></strong> the variance of the first sample and
      * <strong><code>var2</code></strong> the variance of the second sample.
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The datasets described by the two Univariates must each contain
      * at least 2 observations.
      * </li></ul>
      *
      * @param sampleStats1 StatisticalSummary describing data from the first sample
      * @param sampleStats2 StatisticalSummary describing data from the second sample
      * @return t statistic
      * @throws NullArgumentException if the sample statistics are <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      */
     public double homoscedasticT(final StatisticalSummary sampleStats1,
                                  final StatisticalSummary sampleStats2)
         throws MathIllegalArgumentException, NullArgumentException {
 
         checkSampleData(sampleStats1);
         checkSampleData(sampleStats2);
         return homoscedasticT(sampleStats1.getMean(), sampleStats2.getMean(),
                               sampleStats1.getVariance(), sampleStats2.getVariance(),
                               sampleStats1.getN(), sampleStats2.getN());
     }
 
     /**
      * Returns the <i>observed significance level</i>, or
      * <i>p-value</i>, associated with a one-sample, two-tailed t-test
      * comparing the mean of the input array with the constant <code>mu</code>.
      * <p>
      * The number returned is the smallest significance level
      * at which one can reject the null hypothesis that the mean equals
      * <code>mu</code> in favor of the two-sided alternative that the mean
      * is different from <code>mu</code>. For a one-sided test, divide the
      * returned value by 2.</p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">here</a>
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array length must be at least 2.
      * </li></ul>
      *
      * @param mu constant value to compare sample mean against
      * @param sample array of sample data values
      * @return p-value
      * @throws NullArgumentException if the sample array is <code>null</code>
      * @throws MathIllegalArgumentException if the length of the array is &lt; 2
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public double tTest(final double mu, final double[] sample)
         throws MathIllegalArgumentException, NullArgumentException,
         MathIllegalStateException {
 
         checkSampleData(sample);
         // No try-catch or advertised exception because args have just been checked
         return tTest(StatUtils.mean(sample), mu, StatUtils.variance(sample),
                      sample.length);
     }
 
     /**
      * Performs a <a href="http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm">
      * two-sided t-test</a> evaluating the null hypothesis that the mean of the population from
      * which <code>sample</code> is drawn equals <code>mu</code>.
      * <p>
      * Returns <code>true</code> iff the null hypothesis can be
      * rejected with confidence <code>1 - alpha</code>.  To
      * perform a 1-sided test, use <code>alpha * 2</code></p>
      * <p>* <strong>Examples:</strong></p>
      * <ol>
      * <li>To test the (2-sided) hypothesis <code>sample mean = mu </code> at
      * the 95% level, use <br><code>tTest(mu, sample, 0.05) </code>
      * </li>
      * <li>To test the (one-sided) hypothesis <code> sample mean &lt; mu </code>
      * at the 99% level, first verify that the measured sample mean is less
      * than <code>mu</code> and then use
      * <br><code>tTest(mu, sample, 0.02) </code>
      * </li></ol>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the one-sample
      * parametric t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/sg_glos.html#one-sample">here</a>
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array length must be at least 2.
      * </li></ul>
      *
      * @param mu constant value to compare sample mean against
      * @param sample array of sample data values
      * @param alpha significance level of the test
      * @return p-value
      * @throws NullArgumentException if the sample array is <code>null</code>
      * @throws MathIllegalArgumentException if the length of the array is &lt; 2
      * @throws MathIllegalArgumentException if <code>alpha</code> is not in the range (0, 0.5]
      * @throws MathIllegalStateException if an error computing the p-value
      */
     public boolean tTest(final double mu, final double[] sample, final double alpha)
         throws MathIllegalArgumentException, NullArgumentException, MathIllegalStateException {
 
         checkSignificanceLevel(alpha);
         return tTest(mu, sample) < alpha;
     }
 
     /**
      * Returns the <i>observed significance level</i>, or
      * <i>p-value</i>, associated with a one-sample, two-tailed t-test
      * comparing the mean of the dataset described by <code>sampleStats</code>
      * with the constant <code>mu</code>.
      * <p>
      * The number returned is the smallest significance level
      * at which one can reject the null hypothesis that the mean equals
      * <code>mu</code> in favor of the two-sided alternative that the mean
      * is different from <code>mu</code>. For a one-sided test, divide the
      * returned value by 2.</p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The sample must contain at least 2 observations.
      * </li></ul>
      *
      * @param mu constant value to compare sample mean against
      * @param sampleStats StatisticalSummary describing sample data
      * @return p-value
      * @throws NullArgumentException if <code>sampleStats</code> is <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public double tTest(final double mu, final StatisticalSummary sampleStats)
         throws MathIllegalArgumentException, NullArgumentException,
         MathIllegalStateException {
 
         checkSampleData(sampleStats);
         return tTest(sampleStats.getMean(), mu, sampleStats.getVariance(),
                      sampleStats.getN());
     }
 
     /**
      * Performs a <a href="http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm">
      * two-sided t-test</a> evaluating the null hypothesis that the mean of the
      * population from which the dataset described by <code>stats</code> is
      * drawn equals <code>mu</code>.
      * <p>
      * Returns <code>true</code> iff the null hypothesis can be rejected with
      * confidence <code>1 - alpha</code>.  To  perform a 1-sided test, use
      * <code>alpha * 2.</code></p>
      * <p>* <strong>Examples:</strong></p>
      * <ol>
      * <li>To test the (2-sided) hypothesis <code>sample mean = mu </code> at
      * the 95% level, use <br><code>tTest(mu, sampleStats, 0.05) </code>
      * </li>
      * <li>To test the (one-sided) hypothesis <code> sample mean &lt; mu </code>
      * at the 99% level, first verify that the measured sample mean is less
      * than <code>mu</code> and then use
      * <br><code>tTest(mu, sampleStats, 0.02) </code>
      * </li></ol>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the one-sample
      * parametric t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/sg_glos.html#one-sample">here</a>
      * </p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The sample must include at least 2 observations.
      * </li></ul>
      *
      * @param mu constant value to compare sample mean against
      * @param sampleStats StatisticalSummary describing sample data values
      * @param alpha significance level of the test
      * @return p-value
      * @throws NullArgumentException if <code>sampleStats</code> is <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      * @throws MathIllegalArgumentException if <code>alpha</code> is not in the range (0, 0.5]
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public boolean tTest(final double mu, final StatisticalSummary sampleStats,
                          final double alpha)
         throws MathIllegalArgumentException, NullArgumentException,
                MathIllegalStateException {
 
         checkSignificanceLevel(alpha);
         return tTest(mu, sampleStats) < alpha;
     }
 
     /**
      * Returns the <i>observed significance level</i>, or
      * <i>p-value</i>, associated with a two-sample, two-tailed t-test
      * comparing the means of the input arrays.
      * <p>
      * The number returned is the smallest significance level
      * at which one can reject the null hypothesis that the two means are
      * equal in favor of the two-sided alternative that they are different.
      * For a one-sided test, divide the returned value by 2.</p>
      * <p>
      * The test does not assume that the underlying popuation variances are
      * equal  and it uses approximated degrees of freedom computed from the
      * sample data to compute the p-value.  The t-statistic used is as defined in
      * {@link #t(double[], double[])} and the Welch-Satterthwaite approximation
      * to the degrees of freedom is used,
      * as described
      * <a href="http://www.itl.nist.gov/div898/handbook/prc/section3/prc31.htm">
      * here.</a>  To perform the test under the assumption of equal subpopulation
      * variances, use {@link #homoscedasticTTest(double[], double[])}.</p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the p-value depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array lengths must both be at least 2.
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @return p-value for t-test
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public double tTest(final double[] sample1, final double[] sample2)
         throws MathIllegalArgumentException, NullArgumentException,
                MathIllegalStateException {
 
         checkSampleData(sample1);
         checkSampleData(sample2);
         // No try-catch or advertised exception because args have just been checked
         return tTest(StatUtils.mean(sample1), StatUtils.mean(sample2),
                      StatUtils.variance(sample1), StatUtils.variance(sample2),
                      sample1.length, sample2.length);
     }
 
     /**
      * Returns the <i>observed significance level</i>, or
      * <i>p-value</i>, associated with a two-sample, two-tailed t-test
      * comparing the means of the input arrays, under the assumption that
      * the two samples are drawn from subpopulations with equal variances.
      * To perform the test without the equal variances assumption, use
      * {@link #tTest(double[], double[])}.
      * <p>
      * The number returned is the smallest significance level
      * at which one can reject the null hypothesis that the two means are
      * equal in favor of the two-sided alternative that they are different.
      * For a one-sided test, divide the returned value by 2.</p>
      * <p>
      * A pooled variance estimate is used to compute the t-statistic.  See
      * {@link #homoscedasticT(double[], double[])}. The sum of the sample sizes
      * minus 2 is used as the degrees of freedom.</p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the p-value depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array lengths must both be at least 2.
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @return p-value for t-test
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public double homoscedasticTTest(final double[] sample1, final double[] sample2)
         throws MathIllegalArgumentException, NullArgumentException,
         MathIllegalStateException {
 
         checkSampleData(sample1);
         checkSampleData(sample2);
         // No try-catch or advertised exception because args have just been checked
         return homoscedasticTTest(StatUtils.mean(sample1),
                                   StatUtils.mean(sample2),
                                   StatUtils.variance(sample1),
                                   StatUtils.variance(sample2),
                                   sample1.length, sample2.length);
     }
 
     /**
      * Performs a
      * <a href="http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm">
      * two-sided t-test</a> evaluating the null hypothesis that <code>sample1</code>
      * and <code>sample2</code> are drawn from populations with the same mean,
      * with significance level <code>alpha</code>.  This test does not assume
      * that the subpopulation variances are equal.  To perform the test assuming
      * equal variances, use
      * {@link #homoscedasticTTest(double[], double[], double)}.
      * <p>
      * Returns <code>true</code> iff the null hypothesis that the means are
      * equal can be rejected with confidence <code>1 - alpha</code>.  To
      * perform a 1-sided test, use <code>alpha * 2</code></p>
      * <p>
      * See {@link #t(double[], double[])} for the formula used to compute the
      * t-statistic.  Degrees of freedom are approximated using the
      * <a href="http://www.itl.nist.gov/div898/handbook/prc/section3/prc31.htm">
      * Welch-Satterthwaite approximation.</a></p>
      * <p>* <strong>Examples:</strong></p>
      * <ol>
      * <li>To test the (2-sided) hypothesis <code>mean 1 = mean 2 </code> at
      * the 95% level,  use
      * <br><code>tTest(sample1, sample2, 0.05). </code>
      * </li>
      * <li>To test the (one-sided) hypothesis <code> mean 1 &lt; mean 2 </code>,
      * at the 99% level, first verify that the measured  mean of <code>sample 1</code>
      * is less than the mean of <code>sample 2</code> and then use
      * <br><code>tTest(sample1, sample2, 0.02) </code>
      * </li></ol>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array lengths must both be at least 2.
      * </li>
      * <li> <code> 0 &lt; alpha &lt; 0.5 </code>
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @param alpha significance level of the test
      * @return true if the null hypothesis can be rejected with
      * confidence 1 - alpha
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      * @throws MathIllegalArgumentException if <code>alpha</code> is not in the range (0, 0.5]
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public boolean tTest(final double[] sample1, final double[] sample2,
                          final double alpha)
         throws MathIllegalArgumentException, NullArgumentException, MathIllegalStateException {
 
         checkSignificanceLevel(alpha);
         return tTest(sample1, sample2) < alpha;
     }
 
     /**
      * Performs a
      * <a href="http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm">
      * two-sided t-test</a> evaluating the null hypothesis that <code>sample1</code>
      * and <code>sample2</code> are drawn from populations with the same mean,
      * with significance level <code>alpha</code>,  assuming that the
      * subpopulation variances are equal.  Use
      * {@link #tTest(double[], double[], double)} to perform the test without
      * the assumption of equal variances.
      * <p>
      * Returns <code>true</code> iff the null hypothesis that the means are
      * equal can be rejected with confidence <code>1 - alpha</code>.  To
      * perform a 1-sided test, use <code>alpha * 2.</code>  To perform the test
      * without the assumption of equal subpopulation variances, use
      * {@link #tTest(double[], double[], double)}.</p>
      * <p>
      * A pooled variance estimate is used to compute the t-statistic. See
      * {@link #t(double[], double[])} for the formula. The sum of the sample
      * sizes minus 2 is used as the degrees of freedom.</p>
      * <p><strong>Examples:</strong></p>
      * <ol>
      * <li>To test the (2-sided) hypothesis <code>mean 1 = mean 2 </code> at
      * the 95% level, use <br><code>tTest(sample1, sample2, 0.05). </code>
      * </li>
      * <li>To test the (one-sided) hypothesis <code> mean 1 &lt; mean 2, </code>
      * at the 99% level, first verify that the measured mean of
      * <code>sample 1</code> is less than the mean of <code>sample 2</code>
      * and then use
      * <br><code>tTest(sample1, sample2, 0.02) </code>
      * </li></ol>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The observed array lengths must both be at least 2.
      * </li>
      * <li> <code> 0 &lt; alpha &lt; 0.5 </code>
      * </li></ul>
      *
      * @param sample1 array of sample data values
      * @param sample2 array of sample data values
      * @param alpha significance level of the test
      * @return true if the null hypothesis can be rejected with
      * confidence 1 - alpha
      * @throws NullArgumentException if the arrays are <code>null</code>
      * @throws MathIllegalArgumentException if the length of the arrays is &lt; 2
      * @throws MathIllegalArgumentException if <code>alpha</code> is not in the range (0, 0.5]
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public boolean homoscedasticTTest(final double[] sample1, final double[] sample2,
                                       final double alpha)
         throws MathIllegalArgumentException, NullArgumentException, MathIllegalStateException {
 
         checkSignificanceLevel(alpha);
         return homoscedasticTTest(sample1, sample2) < alpha;
     }
 
     /**
      * Returns the <i>observed significance level</i>, or
      * <i>p-value</i>, associated with a two-sample, two-tailed t-test
      * comparing the means of the datasets described by two StatisticalSummary
      * instances.
      * <p>
      * The number returned is the smallest significance level
      * at which one can reject the null hypothesis that the two means are
      * equal in favor of the two-sided alternative that they are different.
      * For a one-sided test, divide the returned value by 2.</p>
      * <p>
      * The test does not assume that the underlying population variances are
      * equal  and it uses approximated degrees of freedom computed from the
      * sample data to compute the p-value.   To perform the test assuming
      * equal variances, use
      * {@link #homoscedasticTTest(StatisticalSummary, StatisticalSummary)}.</p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the p-value depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The datasets described by the two Univariates must each contain
      * at least 2 observations.
      * </li></ul>
      *
      * @param sampleStats1  StatisticalSummary describing data from the first sample
      * @param sampleStats2  StatisticalSummary describing data from the second sample
      * @return p-value for t-test
      * @throws NullArgumentException if the sample statistics are <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public double tTest(final StatisticalSummary sampleStats1,
                         final StatisticalSummary sampleStats2)
         throws MathIllegalArgumentException, NullArgumentException,
         MathIllegalStateException {
 
         checkSampleData(sampleStats1);
         checkSampleData(sampleStats2);
         return tTest(sampleStats1.getMean(), sampleStats2.getMean(),
                      sampleStats1.getVariance(), sampleStats2.getVariance(),
                      sampleStats1.getN(), sampleStats2.getN());
     }
 
     /**
      * Returns the <i>observed significance level</i>, or
      * <i>p-value</i>, associated with a two-sample, two-tailed t-test
      * comparing the means of the datasets described by two StatisticalSummary
      * instances, under the hypothesis of equal subpopulation variances. To
      * perform a test without the equal variances assumption, use
      * {@link #tTest(StatisticalSummary, StatisticalSummary)}.
      * <p>
      * The number returned is the smallest significance level
      * at which one can reject the null hypothesis that the two means are
      * equal in favor of the two-sided alternative that they are different.
      * For a one-sided test, divide the returned value by 2.</p>
      * <p>
      * See {@link #homoscedasticT(double[], double[])} for the formula used to
      * compute the t-statistic. The sum of the  sample sizes minus 2 is used as
      * the degrees of freedom.</p>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the p-value depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">here</a>
      * </p><p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The datasets described by the two Univariates must each contain
      * at least 2 observations.
      * </li></ul>
      *
      * @param sampleStats1  StatisticalSummary describing data from the first sample
      * @param sampleStats2  StatisticalSummary describing data from the second sample
      * @return p-value for t-test
      * @throws NullArgumentException if the sample statistics are <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public double homoscedasticTTest(final StatisticalSummary sampleStats1,
                                      final StatisticalSummary sampleStats2)
         throws MathIllegalArgumentException, NullArgumentException,
         MathIllegalStateException {
 
         checkSampleData(sampleStats1);
         checkSampleData(sampleStats2);
         return homoscedasticTTest(sampleStats1.getMean(),
                                   sampleStats2.getMean(),
                                   sampleStats1.getVariance(),
                                   sampleStats2.getVariance(),
                                   sampleStats1.getN(), sampleStats2.getN());
     }
 
     /**
      * Performs a
      * <a href="http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm">
      * two-sided t-test</a> evaluating the null hypothesis that
      * <code>sampleStats1</code> and <code>sampleStats2</code> describe
      * datasets drawn from populations with the same mean, with significance
      * level <code>alpha</code>.   This test does not assume that the
      * subpopulation variances are equal.  To perform the test under the equal
      * variances assumption, use
      * {@link #homoscedasticTTest(StatisticalSummary, StatisticalSummary)}.
      * <p>
      * Returns <code>true</code> iff the null hypothesis that the means are
      * equal can be rejected with confidence <code>1 - alpha</code>.  To
      * perform a 1-sided test, use <code>alpha * 2</code></p>
      * <p>
      * See {@link #t(double[], double[])} for the formula used to compute the
      * t-statistic.  Degrees of freedom are approximated using the
      * <a href="http://www.itl.nist.gov/div898/handbook/prc/section3/prc31.htm">
      * Welch-Satterthwaite approximation.</a></p>
      * <p>* <strong>Examples:</strong></p>
      * <ol>
      * <li>To test the (2-sided) hypothesis <code>mean 1 = mean 2 </code> at
      * the 95%, use
      * <br><code>tTest(sampleStats1, sampleStats2, 0.05) </code>
      * </li>
      * <li>To test the (one-sided) hypothesis <code> mean 1 &lt; mean 2 </code>
      * at the 99% level,  first verify that the measured mean of
      * <code>sample 1</code> is less than  the mean of <code>sample 2</code>
      * and then use
      * <br><code>tTest(sampleStats1, sampleStats2, 0.02) </code>
      * </li></ol>
      * <p>
      * <strong>Usage Note:</strong><br>
      * The validity of the test depends on the assumptions of the parametric
      * t-test procedure, as discussed
      * <a href="http://www.basic.nwu.edu/statguidefiles/ttest_unpaired_ass_viol.html">
      * here</a></p>
      * <p><strong>Preconditions</strong>:</p>
      * <ul>
      * <li>The datasets described by the two Univariates must each contain
      * at least 2 observations.
      * </li>
      * <li> <code> 0 &lt; alpha &lt; 0.5 </code>
      * </li></ul>
      *
      * @param sampleStats1 StatisticalSummary describing sample data values
      * @param sampleStats2 StatisticalSummary describing sample data values
      * @param alpha significance level of the test
      * @return true if the null hypothesis can be rejected with
      * confidence 1 - alpha
      * @throws NullArgumentException if the sample statistics are <code>null</code>
      * @throws MathIllegalArgumentException if the number of samples is &lt; 2
      * @throws MathIllegalArgumentException if <code>alpha</code> is not in the range (0, 0.5]
      * @throws MathIllegalStateException if an error occurs computing the p-value
      */
     public boolean tTest(final StatisticalSummary sampleStats1,
                          final StatisticalSummary sampleStats2,
                          final double alpha)
         throws MathIllegalArgumentException, NullArgumentException, MathIllegalStateException {
 
         checkSignificanceLevel(alpha);
         return tTest(sampleStats1, sampleStats2) < alpha;
     }
 
     //----------------------------------------------- Protected methods
 
     /**
      * Computes approximate degrees of freedom for 2-sample t-test.
      *
      * @param v1 first sample variance
      * @param v2 second sample variance
      * @param n1 first sample n
      * @param n2 second sample n
      * @return approximate degrees of freedom
      */
     protected double df(double v1, double v2, double n1, double n2) {
         return (((v1 / n1) + (v2 / n2)) * ((v1 / n1) + (v2 / n2))) /
         ((v1 * v1) / (n1 * n1 * (n1 - 1d)) + (v2 * v2) /
                 (n2 * n2 * (n2 - 1d)));
     }
 
     /**
      * Computes t test statistic for 1-sample t-test.
      *
      * @param m sample mean
      * @param mu constant to test against
      * @param v sample variance
      * @param n sample n
      * @return t test statistic
      */
     protected double t(final double m, final double mu,
                        final double v, final double n) {
         return (m - mu) / FastMath.sqrt(v / n);
     }
 
     /**
      * Computes t test statistic for 2-sample t-test.
      * <p>
      * Does not assume that subpopulation variances are equal.</p>
      *
      * @param m1 first sample mean
      * @param m2 second sample mean
      * @param v1 first sample variance
      * @param v2 second sample variance
      * @param n1 first sample n
      * @param n2 second sample n
      * @return t test statistic
      */
     protected double t(final double m1, final double m2,
                        final double v1, final double v2,
                        final double n1, final double n2)  {
         return (m1 - m2) / FastMath.sqrt((v1 / n1) + (v2 / n2));
     }
 
     /**
      * Computes t test statistic for 2-sample t-test under the hypothesis
      * of equal subpopulation variances.
      *
      * @param m1 first sample mean
      * @param m2 second sample mean
      * @param v1 first sample variance
      * @param v2 second sample variance
      * @param n1 first sample n
      * @param n2 second sample n
      * @return t test statistic
      */
     protected double homoscedasticT(final double m1, final double m2,
                                     final double v1, final double v2,
                                     final double n1, final double n2)  {
         final double pooledVariance = ((n1  - 1) * v1 + (n2 -1) * v2 ) / (n1 + n2 - 2);
         return (m1 - m2) / FastMath.sqrt(pooledVariance * (1d / n1 + 1d / n2));
     }
 
     /**
      * Computes p-value for 2-sided, 1-sample t-test.
      *
      * @param m sample mean
      * @param mu constant to test against
      * @param v sample variance
      * @param n sample n
      * @return p-value
      * @throws MathIllegalStateException if an error occurs computing the p-value
      * @throws MathIllegalArgumentException if n is not greater than 1
      */
     protected double tTest(final double m, final double mu,
                            final double v, final double n)
         throws MathIllegalArgumentException, MathIllegalStateException {
 
         final double t = FastMath.abs(t(m, mu, v, n));
         final TDistribution distribution = new TDistribution( n - 1);
         return 2.0 * distribution.cumulativeProbability(-t);
 
     }
 
     /**
      * Computes p-value for 2-sided, 2-sample t-test.
      * <p>
      * Does not assume subpopulation variances are equal. Degrees of freedom
      * are estimated from the data.</p>
      *
      * @param m1 first sample mean
      * @param m2 second sample mean
      * @param v1 first sample variance
      * @param v2 second sample variance
      * @param n1 first sample n
      * @param n2 second sample n
      * @return p-value
      * @throws MathIllegalStateException if an error occurs computing the p-value
      * @throws MathIllegalArgumentException if the estimated degrees of freedom is not
      * strictly positive
      */
     protected double tTest(final double m1, final double m2,
                            final double v1, final double v2,
                            final double n1, final double n2)
         throws MathIllegalArgumentException, MathIllegalStateException {
 
         final double t = FastMath.abs(t(m1, m2, v1, v2, n1, n2));
         final double degreesOfFreedom = df(v1, v2, n1, n2);
         final TDistribution distribution = new TDistribution(degreesOfFreedom);
         return 2.0 * distribution.cumulativeProbability(-t);
 
     }
 
     /**
      * Computes p-value for 2-sided, 2-sample t-test, under the assumption
      * of equal subpopulation variances.
      * <p>
      * The sum of the sample sizes minus 2 is used as degrees of freedom.</p>
      *
      * @param m1 first sample mean
      * @param m2 second sample mean
      * @param v1 first sample variance
      * @param v2 second sample variance
      * @param n1 first sample n
      * @param n2 second sample n
      * @return p-value
      * @throws MathIllegalStateException if an error occurs computing the p-value
      * @throws MathIllegalArgumentException if the estimated degrees of freedom is not
      * strictly positive
      */
     protected double homoscedasticTTest(double m1, double m2,
                                         double v1, double v2,
                                         double n1, double n2)
         throws MathIllegalArgumentException, MathIllegalStateException {
 
         final double t = FastMath.abs(homoscedasticT(m1, m2, v1, v2, n1, n2));
         final double degreesOfFreedom = n1 + n2 - 2;
         final TDistribution distribution = new TDistribution(degreesOfFreedom);
         return 2.0 * distribution.cumulativeProbability(-t);
 
     }
 
     /**
      * Check significance level.
      *
      * @param alpha significance level
      * @throws MathIllegalArgumentException if the significance level is out of bounds.
      */
     private void checkSignificanceLevel(final double alpha)
         throws MathIllegalArgumentException {
 
         if (alpha <= 0 || alpha > 0.5) {
             throw new MathIllegalArgumentException(LocalizedStatFormats.SIGNIFICANCE_LEVEL,
                                           alpha, 0.0, 0.5);
         }
 
     }
 
     /**
      * Check sample data.
      *
      * @param data Sample data.
      * @throws NullArgumentException if {@code data} is {@code null}.
      * @throws MathIllegalArgumentException if there is not enough sample data.
      */
     private void checkSampleData(final double[] data)
         throws MathIllegalArgumentException, NullArgumentException {
 
         MathUtils.checkNotNull(data, LocalizedCoreFormats.INPUT_ARRAY);
         if (data.length < 2) {
             throw new MathIllegalArgumentException(
                     LocalizedStatFormats.INSUFFICIENT_DATA_FOR_T_STATISTIC,
                     data.length, 2, true);
         }
 
     }
 
     /**
      * Check sample data.
      *
      * @param stat Statistical summary.
      * @throws NullArgumentException if {@code data} is {@code null}.
      * @throws MathIllegalArgumentException if there is not enough sample data.
      */
     private void checkSampleData(final StatisticalSummary stat)
         throws MathIllegalArgumentException, NullArgumentException {
 
         MathUtils.checkNotNull(stat);
         if (stat.getN() < 2) {
             throw new MathIllegalArgumentException(
                     LocalizedStatFormats.INSUFFICIENT_DATA_FOR_T_STATISTIC,
                     stat.getN(), 2, true);
         }
 
     }
 
 }