org.apache.commons.math3.stat.inference

## Class TTest

- java.lang.Object
- - org.apache.commons.math3.stat.inference.TTest

-     public class TTest
      extends Object

  An implementation for Student's t-tests.

  Tests can be:

  - One-sample or two-sample
  - One-sided or two-sided
  - Paired or unpaired (for two-sample tests)
  - Homoscedastic (equal variance assumption) or heteroscedastic (for two sample tests)
  - Fixed significance level (boolean-valued) or returning p-values.

  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, `double-`valued methods return p-values; `boolean-`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 `alpha=0.05`).

  Input to tests can be either `double[]` arrays or `StatisticalSummary` instances.

  Uses commons-math `TDistribution` implementation to estimate exact p-values.

  Version:
    $Id: TTest.java 1416643 2012-12-03 19:37:14Z tn $

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `TTest()`

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `private void` | `checkSampleData(double[] data)` Check sample data.
    `private void` | `checkSampleData(StatisticalSummary stat)` Check sample data.
    `private void` | `checkSignificanceLevel(double alpha)` Check significance level.
    `protected double` | `df(double v1, double v2, double n1, double n2)` Computes approximate degrees of freedom for 2-sample t-test.
    `double` | `homoscedasticT(double[] sample1, double[] sample2)` Computes a 2-sample t statistic, under the hypothesis of equal subpopulation variances.
    `protected double` | `homoscedasticT(double m1, double m2, double v1, double v2, double n1, double n2)` Computes t test statistic for 2-sample t-test under the hypothesis of equal subpopulation variances.
    `double` | `homoscedasticT(StatisticalSummary sampleStats1, StatisticalSummary sampleStats2)` Computes a 2-sample t statistic, comparing the means of the datasets described by two `StatisticalSummary` instances, under the assumption of equal subpopulation variances.
    `double` | `homoscedasticTTest(double[] sample1, double[] sample2)` Returns the observed significance level, or p-value, 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.
    `boolean` | `homoscedasticTTest(double[] sample1, double[] sample2, double alpha)` Performs a two-sided t-test evaluating the null hypothesis that `sample1` and `sample2` are drawn from populations with the same mean, with significance level `alpha`, assuming that the subpopulation variances are equal.
    `protected double` | `homoscedasticTTest(double m1, double m2, double v1, double v2, double n1, double n2)` Computes p-value for 2-sided, 2-sample t-test, under the assumption of equal subpopulation variances.
    `double` | `homoscedasticTTest(StatisticalSummary sampleStats1, StatisticalSummary sampleStats2)` Returns the observed significance level, or p-value, 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.
    `double` | `pairedT(double[] sample1, double[] sample2)` Computes a paired, 2-sample t-statistic based on the data in the input arrays.
    `double` | `pairedTTest(double[] sample1, double[] sample2)` Returns the observed significance level, or p-value, associated with a paired, two-sample, two-tailed t-test based on the data in the input arrays.
    `boolean` | `pairedTTest(double[] sample1, double[] sample2, double alpha)` Performs a paired t-test evaluating the null hypothesis that the mean of the paired differences between `sample1` and `sample2` is 0 in favor of the two-sided alternative that the mean paired difference is not equal to 0, with significance level `alpha`.
    `double` | `t(double[] sample1, double[] sample2)` Computes a 2-sample t statistic, without the hypothesis of equal subpopulation variances.
    `double` | `t(double mu, double[] observed)` Computes a t statistic given observed values and a comparison constant.
    `protected double` | `t(double m, double mu, double v, double n)` Computes t test statistic for 1-sample t-test.
    `protected double` | `t(double m1, double m2, double v1, double v2, double n1, double n2)` Computes t test statistic for 2-sample t-test.
    `double` | `t(double mu, StatisticalSummary sampleStats)` Computes a t statistic to use in comparing the mean of the dataset described by `sampleStats` to `mu`.
    `double` | `t(StatisticalSummary sampleStats1, StatisticalSummary sampleStats2)` Computes a 2-sample t statistic , comparing the means of the datasets described by two `StatisticalSummary` instances, without the assumption of equal subpopulation variances.
    `double` | `tTest(double[] sample1, double[] sample2)` Returns the observed significance level, or p-value, associated with a two-sample, two-tailed t-test comparing the means of the input arrays.
    `boolean` | `tTest(double[] sample1, double[] sample2, double alpha)` Performs a two-sided t-test evaluating the null hypothesis that `sample1` and `sample2` are drawn from populations with the same mean, with significance level `alpha`.
    `double` | `tTest(double mu, double[] sample)` Returns the observed significance level, or p-value, associated with a one-sample, two-tailed t-test comparing the mean of the input array with the constant `mu`.
    `boolean` | `tTest(double mu, double[] sample, double alpha)` Performs a two-sided t-test evaluating the null hypothesis that the mean of the population from which `sample` is drawn equals `mu`.
    `protected double` | `tTest(double m, double mu, double v, double n)` Computes p-value for 2-sided, 1-sample t-test.
    `protected double` | `tTest(double m1, double m2, double v1, double v2, double n1, double n2)` Computes p-value for 2-sided, 2-sample t-test.
    `double` | `tTest(double mu, StatisticalSummary sampleStats)` Returns the observed significance level, or p-value, associated with a one-sample, two-tailed t-test comparing the mean of the dataset described by `sampleStats` with the constant `mu`.
    `boolean` | `tTest(double mu, StatisticalSummary sampleStats, double alpha)` Performs a two-sided t-test evaluating the null hypothesis that the mean of the population from which the dataset described by `stats` is drawn equals `mu`.
    `double` | `tTest(StatisticalSummary sampleStats1, StatisticalSummary sampleStats2)` Returns the observed significance level, or p-value, associated with a two-sample, two-tailed t-test comparing the means of the datasets described by two StatisticalSummary instances.
    `boolean` | `tTest(StatisticalSummary sampleStats1, StatisticalSummary sampleStats2, double alpha)` Performs a two-sided t-test evaluating the null hypothesis that `sampleStats1` and `sampleStats2` describe datasets drawn from populations with the same mean, with significance level `alpha`.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### TTest

          public TTest()

  - ### Method Detail

    - #### pairedT

          public double pairedT(double[] sample1,
                       double[] sample2)
                         throws NullArgumentException,
                                NoDataException,
                                DimensionMismatchException,
                                NumberIsTooSmallException

      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 `t(double, double[])`, with `mu = 0` and the sample array consisting of the (signed) differences between corresponding entries in `sample1` and `sample2.`

      Preconditions:

      - The input arrays must have the same length and their common length must be at least 2.
      Parameters:
        `sample1` - array of sample data values
        `sample2` - array of sample data values
      Returns:
        t statistic
      Throws:
        `NullArgumentException` - if the arrays are `null`
        `NoDataException` - if the arrays are empty
        `DimensionMismatchException` - if the length of the arrays is not equal
        `NumberIsTooSmallException` - if the length of the arrays is < 2

    - #### pairedTTest

          public double pairedTTest(double[] sample1,
                           double[] sample2)
                             throws NullArgumentException,
                                    NoDataException,
                                    DimensionMismatchException,
                                    NumberIsTooSmallException,
                                    MaxCountExceededException

      Returns the observed significance level, or p-value, associated with a paired, two-sample, two-tailed t-test based on the data in the input arrays.

      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.

      This test is equivalent to a one-sample t-test computed using `tTest(double, double[])` with `mu = 0` and the sample array consisting of the signed differences between corresponding elements of `sample1` and `sample2.`

      Usage Note:
      The validity of the p-value depends on the assumptions of the parametric t-test procedure, as discussed here

      Preconditions:

      - The input array lengths must be the same and their common length must be at least 2.
      Parameters:
        `sample1` - array of sample data values
        `sample2` - array of sample data values
      Returns:
        p-value for t-test
      Throws:
        `NullArgumentException` - if the arrays are `null`
        `NoDataException` - if the arrays are empty
        `DimensionMismatchException` - if the length of the arrays is not equal
        `NumberIsTooSmallException` - if the length of the arrays is < 2
        `MaxCountExceededException` - if an error occurs computing the p-value

    - #### pairedTTest

          public boolean pairedTTest(double[] sample1,
                            double[] sample2,
                            double alpha)
                              throws NullArgumentException,
                                     NoDataException,
                                     DimensionMismatchException,
                                     NumberIsTooSmallException,
                                     OutOfRangeException,
                                     MaxCountExceededException

      Performs a paired t-test evaluating the null hypothesis that the mean of the paired differences between `sample1` and `sample2` is 0 in favor of the two-sided alternative that the mean paired difference is not equal to 0, with significance level `alpha`.

      Returns `true` iff the null hypothesis can be rejected with confidence `1 - alpha`. To perform a 1-sided test, use `alpha * 2`

      Usage Note:
      The validity of the test depends on the assumptions of the parametric t-test procedure, as discussed here

      Preconditions:

      - The input array lengths must be the same and their common length must be at least 2.
      - `0 < alpha < 0.5`
      Parameters:
        `sample1` - array of sample data values
        `sample2` - array of sample data values
        `alpha` - significance level of the test
      Returns:
        true if the null hypothesis can be rejected with confidence 1 - alpha
      Throws:
        `NullArgumentException` - if the arrays are `null`
        `NoDataException` - if the arrays are empty
        `DimensionMismatchException` - if the length of the arrays is not equal
        `NumberIsTooSmallException` - if the length of the arrays is < 2
        `OutOfRangeException` - if `alpha` is not in the range (0, 0.5]
        `MaxCountExceededException` - if an error occurs computing the p-value

    - #### t

          public double t(double mu,
                 double[] observed)
                   throws NullArgumentException,
                          NumberIsTooSmallException

      Computes a t statistic given observed values and a comparison constant.

      This statistic can be used to perform a one sample t-test for the mean.

      Preconditions:

      - The observed array length must be at least 2.
      Parameters:
        `mu` - comparison constant
        `observed` - array of values
      Returns:
        t statistic
      Throws:
        `NullArgumentException` - if `observed` is `null`
        `NumberIsTooSmallException` - if the length of `observed` is < 2

    - #### t

          public double t(double mu,
                 StatisticalSummary sampleStats)
                   throws NullArgumentException,
                          NumberIsTooSmallException

      Computes a t statistic to use in comparing the mean of the dataset described by `sampleStats` to `mu`.

      This statistic can be used to perform a one sample t-test for the mean.

      Preconditions:

      - `observed.getN() ≥ 2`.
      Parameters:
        `mu` - comparison constant
        `sampleStats` - DescriptiveStatistics holding sample summary statitstics
      Returns:
        t statistic
      Throws:
        `NullArgumentException` - if `sampleStats` is `null`
        `NumberIsTooSmallException` - if the number of samples is < 2

    - #### homoscedasticT

          public double homoscedasticT(double[] sample1,
                              double[] sample2)
                                throws NullArgumentException,
                                       NumberIsTooSmallException

      Computes a 2-sample t statistic, under the hypothesis of equal subpopulation variances. To compute a t-statistic without the equal variances hypothesis, use `t(double[], double[])`.

      This statistic can be used to perform a (homoscedastic) two-sample t-test to compare sample means.

      The t-statistic is

      `t = (m1 - m2) / (sqrt(1/n1 +1/n2) sqrt(var))`

      where `n1` is the size of first sample; `n2` is the size of second sample; `m1` is the mean of first sample; `m2` is the mean of second sample

    and `var` is the pooled variance estimate:

    `var = sqrt(((n1 - 1)var1 + (n2 - 1)var2) / ((n1-1) + (n2-1)))`

    with `var1` the variance of the first sample and `var2` the variance of the second sample.

    Preconditions:

    - The observed array lengths must both be at least 2.

Parameters:
  `sample1` - array of sample data values
  `sample2` - array of sample data values
Returns:
  t statistic
Throws:
  `NullArgumentException` - if the arrays are `null`
  `NumberIsTooSmallException` - if the length of the arrays is < 2

- #### t

      public double t(double[] sample1,
             double[] sample2)
               throws NullArgumentException,
                      NumberIsTooSmallException

  Computes a 2-sample t statistic, without the hypothesis of equal subpopulation variances. To compute a t-statistic assuming equal variances, use `homoscedasticT(double[], double[])`.

  This statistic can be used to perform a two-sample t-test to compare sample means.

  The t-statistic is

  `t = (m1 - m2) / sqrt(var1/n1 + var2/n2)`

  where `n1` is the size of the first sample `n2` is the size of the second sample; `m1` is the mean of the first sample; `m2` is the mean of the second sample; `var1` is the variance of the first sample; `var2` is the variance of the second sample;

  Preconditions:

  - The observed array lengths must both be at least 2.
  Parameters:
    `sample1` - array of sample data values
    `sample2` - array of sample data values
  Returns:
    t statistic
  Throws:
    `NullArgumentException` - if the arrays are `null`
    `NumberIsTooSmallException` - if the length of the arrays is < 2

- #### t

      public double t(StatisticalSummary sampleStats1,
             StatisticalSummary sampleStats2)
               throws NullArgumentException,
                      NumberIsTooSmallException

  Computes a 2-sample t statistic , comparing the means of the datasets described by two `StatisticalSummary` instances, without the assumption of equal subpopulation variances. Use `homoscedasticT(StatisticalSummary, StatisticalSummary)` to compute a t-statistic under the equal variances assumption.

  This statistic can be used to perform a two-sample t-test to compare sample means.

  The returned t-statistic is

  `t = (m1 - m2) / sqrt(var1/n1 + var2/n2)`

  where `n1` is the size of the first sample; `n2` is the size of the second sample; `m1` is the mean of the first sample; `m2` is the mean of the second sample `var1` is the variance of the first sample; `var2` is the variance of the second sample

  Preconditions:

  - The datasets described by the two Univariates must each contain at least 2 observations.
  Parameters:
    `sampleStats1` - StatisticalSummary describing data from the first sample
    `sampleStats2` - StatisticalSummary describing data from the second sample
  Returns:
    t statistic
  Throws:
    `NullArgumentException` - if the sample statistics are `null`
    `NumberIsTooSmallException` - if the number of samples is < 2

- #### homoscedasticT

      public double homoscedasticT(StatisticalSummary sampleStats1,
                          StatisticalSummary sampleStats2)
                            throws NullArgumentException,
                                   NumberIsTooSmallException

  Computes a 2-sample t statistic, comparing the means of the datasets described by two `StatisticalSummary` instances, under the assumption of equal subpopulation variances. To compute a t-statistic without the equal variances assumption, use `t(StatisticalSummary, StatisticalSummary)`.

  This statistic can be used to perform a (homoscedastic) two-sample t-test to compare sample means.

  The t-statistic returned is

  `t = (m1 - m2) / (sqrt(1/n1 +1/n2) sqrt(var))`

  where `n1` is the size of first sample; `n2` is the size of second sample; `m1` is the mean of first sample; `m2` is the mean of second sample and `var` is the pooled variance estimate:

  `var = sqrt(((n1 - 1)var1 + (n2 - 1)var2) / ((n1-1) + (n2-1)))`

  with `var1` the variance of the first sample and `var2` the variance of the second sample.

  Preconditions:

  - The datasets described by the two Univariates must each contain at least 2 observations.
  Parameters:
    `sampleStats1` - StatisticalSummary describing data from the first sample
    `sampleStats2` - StatisticalSummary describing data from the second sample
  Returns:
    t statistic
  Throws:
    `NullArgumentException` - if the sample statistics are `null`
    `NumberIsTooSmallException` - if the number of samples is < 2

- #### tTest

      public double tTest(double mu,
                 double[] sample)
                   throws NullArgumentException,
                          NumberIsTooSmallException,
                          MaxCountExceededException

  Returns the observed significance level, or p-value, associated with a one-sample, two-tailed t-test comparing the mean of the input array with the constant `mu`.

  The number returned is the smallest significance level at which one can reject the null hypothesis that the mean equals `mu` in favor of the two-sided alternative that the mean is different from `mu`. For a one-sided test, divide the returned value by 2.

  Usage Note:
  The validity of the test depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The observed array length must be at least 2.
  Parameters:
    `mu` - constant value to compare sample mean against
    `sample` - array of sample data values
  Returns:
    p-value
  Throws:
    `NullArgumentException` - if the sample array is `null`
    `NumberIsTooSmallException` - if the length of the array is < 2
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### tTest

      public boolean tTest(double mu,
                  double[] sample,
                  double alpha)
                    throws NullArgumentException,
                           NumberIsTooSmallException,
                           OutOfRangeException,
                           MaxCountExceededException

  Performs a two-sided t-test evaluating the null hypothesis that the mean of the population from which `sample` is drawn equals `mu`.

  Returns `true` iff the null hypothesis can be rejected with confidence `1 - alpha`. To perform a 1-sided test, use `alpha * 2`

  Examples:

  1. To test the (2-sided) hypothesis `sample mean = mu` at the 95% level, use
     `tTest(mu, sample, 0.05)`
  2. To test the (one-sided) hypothesis `sample mean < mu` at the 99% level, first verify that the measured sample mean is less than `mu` and then use
     `tTest(mu, sample, 0.02)`

  Usage Note:
  The validity of the test depends on the assumptions of the one-sample parametric t-test procedure, as discussed here

  Preconditions:

  - The observed array length must be at least 2.
  Parameters:
    `mu` - constant value to compare sample mean against
    `sample` - array of sample data values
    `alpha` - significance level of the test
  Returns:
    p-value
  Throws:
    `NullArgumentException` - if the sample array is `null`
    `NumberIsTooSmallException` - if the length of the array is < 2
    `OutOfRangeException` - if `alpha` is not in the range (0, 0.5]
    `MaxCountExceededException` - if an error computing the p-value

- #### tTest

      public double tTest(double mu,
                 StatisticalSummary sampleStats)
                   throws NullArgumentException,
                          NumberIsTooSmallException,
                          MaxCountExceededException

  Returns the observed significance level, or p-value, associated with a one-sample, two-tailed t-test comparing the mean of the dataset described by `sampleStats` with the constant `mu`.

  The number returned is the smallest significance level at which one can reject the null hypothesis that the mean equals `mu` in favor of the two-sided alternative that the mean is different from `mu`. For a one-sided test, divide the returned value by 2.

  Usage Note:
  The validity of the test depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The sample must contain at least 2 observations.
  Parameters:
    `mu` - constant value to compare sample mean against
    `sampleStats` - StatisticalSummary describing sample data
  Returns:
    p-value
  Throws:
    `NullArgumentException` - if `sampleStats` is `null`
    `NumberIsTooSmallException` - if the number of samples is < 2
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### tTest

      public boolean tTest(double mu,
                  StatisticalSummary sampleStats,
                  double alpha)
                    throws NullArgumentException,
                           NumberIsTooSmallException,
                           OutOfRangeException,
                           MaxCountExceededException

  Performs a two-sided t-test evaluating the null hypothesis that the mean of the population from which the dataset described by `stats` is drawn equals `mu`.

  Returns `true` iff the null hypothesis can be rejected with confidence `1 - alpha`. To perform a 1-sided test, use `alpha * 2.`

  Examples:

  1. To test the (2-sided) hypothesis `sample mean = mu` at the 95% level, use
     `tTest(mu, sampleStats, 0.05)`
  2. To test the (one-sided) hypothesis `sample mean < mu` at the 99% level, first verify that the measured sample mean is less than `mu` and then use
     `tTest(mu, sampleStats, 0.02)`

  Usage Note:
  The validity of the test depends on the assumptions of the one-sample parametric t-test procedure, as discussed here

  Preconditions:

  - The sample must include at least 2 observations.
  Parameters:
    `mu` - constant value to compare sample mean against
    `sampleStats` - StatisticalSummary describing sample data values
    `alpha` - significance level of the test
  Returns:
    p-value
  Throws:
    `NullArgumentException` - if `sampleStats` is `null`
    `NumberIsTooSmallException` - if the number of samples is < 2
    `OutOfRangeException` - if `alpha` is not in the range (0, 0.5]
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### tTest

      public double tTest(double[] sample1,
                 double[] sample2)
                   throws NullArgumentException,
                          NumberIsTooSmallException,
                          MaxCountExceededException

  Returns the observed significance level, or p-value, associated with a two-sample, two-tailed t-test comparing the means of the input arrays.

  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.

  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 `t(double[], double[])` and the Welch-Satterthwaite approximation to the degrees of freedom is used, as described here. To perform the test under the assumption of equal subpopulation variances, use `homoscedasticTTest(double[], double[])`.

  Usage Note:
  The validity of the p-value depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The observed array lengths must both be at least 2.
  Parameters:
    `sample1` - array of sample data values
    `sample2` - array of sample data values
  Returns:
    p-value for t-test
  Throws:
    `NullArgumentException` - if the arrays are `null`
    `NumberIsTooSmallException` - if the length of the arrays is < 2
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### homoscedasticTTest

      public double homoscedasticTTest(double[] sample1,
                              double[] sample2)
                                throws NullArgumentException,
                                       NumberIsTooSmallException,
                                       MaxCountExceededException

  Returns the observed significance level, or p-value, 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 `tTest(double[], double[])`.

  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.

  A pooled variance estimate is used to compute the t-statistic. See `homoscedasticT(double[], double[])`. The sum of the sample sizes minus 2 is used as the degrees of freedom.

  Usage Note:
  The validity of the p-value depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The observed array lengths must both be at least 2.
  Parameters:
    `sample1` - array of sample data values
    `sample2` - array of sample data values
  Returns:
    p-value for t-test
  Throws:
    `NullArgumentException` - if the arrays are `null`
    `NumberIsTooSmallException` - if the length of the arrays is < 2
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### tTest

      public boolean tTest(double[] sample1,
                  double[] sample2,
                  double alpha)
                    throws NullArgumentException,
                           NumberIsTooSmallException,
                           OutOfRangeException,
                           MaxCountExceededException

  Performs a two-sided t-test evaluating the null hypothesis that `sample1` and `sample2` are drawn from populations with the same mean, with significance level `alpha`. This test does not assume that the subpopulation variances are equal. To perform the test assuming equal variances, use `homoscedasticTTest(double[], double[], double)`.

  Returns `true` iff the null hypothesis that the means are equal can be rejected with confidence `1 - alpha`. To perform a 1-sided test, use `alpha * 2`

  See `t(double[], double[])` for the formula used to compute the t-statistic. Degrees of freedom are approximated using the Welch-Satterthwaite approximation.

  Examples:

  1. To test the (2-sided) hypothesis `mean 1 = mean 2` at the 95% level, use
     `tTest(sample1, sample2, 0.05).`
  2. To test the (one-sided) hypothesis `mean 1 < mean 2` , at the 99% level, first verify that the measured mean of `sample 1` is less than the mean of `sample 2` and then use
     `tTest(sample1, sample2, 0.02)`

  Usage Note:
  The validity of the test depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The observed array lengths must both be at least 2.
  - `0 < alpha < 0.5`
  Parameters:
    `sample1` - array of sample data values
    `sample2` - array of sample data values
    `alpha` - significance level of the test
  Returns:
    true if the null hypothesis can be rejected with confidence 1 - alpha
  Throws:
    `NullArgumentException` - if the arrays are `null`
    `NumberIsTooSmallException` - if the length of the arrays is < 2
    `OutOfRangeException` - if `alpha` is not in the range (0, 0.5]
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### homoscedasticTTest

      public boolean homoscedasticTTest(double[] sample1,
                               double[] sample2,
                               double alpha)
                                 throws NullArgumentException,
                                        NumberIsTooSmallException,
                                        OutOfRangeException,
                                        MaxCountExceededException

  Performs a two-sided t-test evaluating the null hypothesis that `sample1` and `sample2` are drawn from populations with the same mean, with significance level `alpha`, assuming that the subpopulation variances are equal. Use `tTest(double[], double[], double)` to perform the test without the assumption of equal variances.

  Returns `true` iff the null hypothesis that the means are equal can be rejected with confidence `1 - alpha`. To perform a 1-sided test, use `alpha * 2.` To perform the test without the assumption of equal subpopulation variances, use `tTest(double[], double[], double)`.

  A pooled variance estimate is used to compute the t-statistic. See `t(double[], double[])` for the formula. The sum of the sample sizes minus 2 is used as the degrees of freedom.

  Examples:

  1. To test the (2-sided) hypothesis `mean 1 = mean 2` at the 95% level, use
     `tTest(sample1, sample2, 0.05).`
  2. To test the (one-sided) hypothesis `mean 1 < mean 2,` at the 99% level, first verify that the measured mean of `sample 1` is less than the mean of `sample 2` and then use
     `tTest(sample1, sample2, 0.02)`

  Usage Note:
  The validity of the test depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The observed array lengths must both be at least 2.
  - `0 < alpha < 0.5`
  Parameters:
    `sample1` - array of sample data values
    `sample2` - array of sample data values
    `alpha` - significance level of the test
  Returns:
    true if the null hypothesis can be rejected with confidence 1 - alpha
  Throws:
    `NullArgumentException` - if the arrays are `null`
    `NumberIsTooSmallException` - if the length of the arrays is < 2
    `OutOfRangeException` - if `alpha` is not in the range (0, 0.5]
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### tTest

      public double tTest(StatisticalSummary sampleStats1,
                 StatisticalSummary sampleStats2)
                   throws NullArgumentException,
                          NumberIsTooSmallException,
                          MaxCountExceededException

  Returns the observed significance level, or p-value, associated with a two-sample, two-tailed t-test comparing the means of the datasets described by two StatisticalSummary instances.

  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.

  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 `homoscedasticTTest(StatisticalSummary, StatisticalSummary)`.

  Usage Note:
  The validity of the p-value depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The datasets described by the two Univariates must each contain at least 2 observations.
  Parameters:
    `sampleStats1` - StatisticalSummary describing data from the first sample
    `sampleStats2` - StatisticalSummary describing data from the second sample
  Returns:
    p-value for t-test
  Throws:
    `NullArgumentException` - if the sample statistics are `null`
    `NumberIsTooSmallException` - if the number of samples is < 2
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### homoscedasticTTest

      public double homoscedasticTTest(StatisticalSummary sampleStats1,
                              StatisticalSummary sampleStats2)
                                throws NullArgumentException,
                                       NumberIsTooSmallException,
                                       MaxCountExceededException

  Returns the observed significance level, or p-value, 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 `tTest(StatisticalSummary, StatisticalSummary)`.

  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.

  See `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.

  Usage Note:
  The validity of the p-value depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The datasets described by the two Univariates must each contain at least 2 observations.
  Parameters:
    `sampleStats1` - StatisticalSummary describing data from the first sample
    `sampleStats2` - StatisticalSummary describing data from the second sample
  Returns:
    p-value for t-test
  Throws:
    `NullArgumentException` - if the sample statistics are `null`
    `NumberIsTooSmallException` - if the number of samples is < 2
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### tTest

      public boolean tTest(StatisticalSummary sampleStats1,
                  StatisticalSummary sampleStats2,
                  double alpha)
                    throws NullArgumentException,
                           NumberIsTooSmallException,
                           OutOfRangeException,
                           MaxCountExceededException

  Performs a two-sided t-test evaluating the null hypothesis that `sampleStats1` and `sampleStats2` describe datasets drawn from populations with the same mean, with significance level `alpha`. This test does not assume that the subpopulation variances are equal. To perform the test under the equal variances assumption, use `homoscedasticTTest(StatisticalSummary, StatisticalSummary)`.

  Returns `true` iff the null hypothesis that the means are equal can be rejected with confidence `1 - alpha`. To perform a 1-sided test, use `alpha * 2`

  See `t(double[], double[])` for the formula used to compute the t-statistic. Degrees of freedom are approximated using the Welch-Satterthwaite approximation.

  Examples:

  1. To test the (2-sided) hypothesis `mean 1 = mean 2` at the 95%, use
     `tTest(sampleStats1, sampleStats2, 0.05)`
  2. To test the (one-sided) hypothesis `mean 1 < mean 2` at the 99% level, first verify that the measured mean of `sample 1` is less than the mean of `sample 2` and then use
     `tTest(sampleStats1, sampleStats2, 0.02)`

  Usage Note:
  The validity of the test depends on the assumptions of the parametric t-test procedure, as discussed here

  Preconditions:

  - The datasets described by the two Univariates must each contain at least 2 observations.
  - `0 < alpha < 0.5`
  Parameters:
    `sampleStats1` - StatisticalSummary describing sample data values
    `sampleStats2` - StatisticalSummary describing sample data values
    `alpha` - significance level of the test
  Returns:
    true if the null hypothesis can be rejected with confidence 1 - alpha
  Throws:
    `NullArgumentException` - if the sample statistics are `null`
    `NumberIsTooSmallException` - if the number of samples is < 2
    `OutOfRangeException` - if `alpha` is not in the range (0, 0.5]
    `MaxCountExceededException` - if an error occurs computing the p-value

- #### df

      protected double df(double v1,
              double v2,
              double n1,
              double n2)

  Computes approximate degrees of freedom for 2-sample t-test.
  Parameters:
    `v1` - first sample variance
    `v2` - second sample variance
    `n1` - first sample n
    `n2` - second sample n
  Returns:
    approximate degrees of freedom

- #### t

      protected double t(double m,
             double mu,
             double v,
             double n)

  Computes t test statistic for 1-sample t-test.
  Parameters:
    `m` - sample mean
    `mu` - constant to test against
    `v` - sample variance
    `n` - sample n
  Returns:
    t test statistic

- #### t

      protected double t(double m1,
             double m2,
             double v1,
             double v2,
             double n1,
             double n2)

  Computes t test statistic for 2-sample t-test.

  Does not assume that subpopulation variances are equal.

  Parameters:
    `m1` - first sample mean
    `m2` - second sample mean
    `v1` - first sample variance
    `v2` - second sample variance
    `n1` - first sample n
    `n2` - second sample n
  Returns:
    t test statistic

- #### homoscedasticT

      protected double homoscedasticT(double m1,
                          double m2,
                          double v1,
                          double v2,
                          double n1,
                          double n2)

  Computes t test statistic for 2-sample t-test under the hypothesis of equal subpopulation variances.
  Parameters:
    `m1` - first sample mean
    `m2` - second sample mean
    `v1` - first sample variance
    `v2` - second sample variance
    `n1` - first sample n
    `n2` - second sample n
  Returns:
    t test statistic

- #### tTest

      protected double tTest(double m,
                 double mu,
                 double v,
                 double n)
                      throws MaxCountExceededException,
                             MathIllegalArgumentException

  Computes p-value for 2-sided, 1-sample t-test.
  Parameters:
    `m` - sample mean
    `mu` - constant to test against
    `v` - sample variance
    `n` - sample n
  Returns:
    p-value
  Throws:
    `MaxCountExceededException` - if an error occurs computing the p-value
    `MathIllegalArgumentException` - if n is not greater than 1

- #### tTest

      protected double tTest(double m1,
                 double m2,
                 double v1,
                 double v2,
                 double n1,
                 double n2)
                      throws MaxCountExceededException,
                             NotStrictlyPositiveException

  Computes p-value for 2-sided, 2-sample t-test.

  Does not assume subpopulation variances are equal. Degrees of freedom are estimated from the data.

  Parameters:
    `m1` - first sample mean
    `m2` - second sample mean
    `v1` - first sample variance
    `v2` - second sample variance
    `n1` - first sample n
    `n2` - second sample n
  Returns:
    p-value
  Throws:
    `MaxCountExceededException` - if an error occurs computing the p-value
    `NotStrictlyPositiveException` - if the estimated degrees of freedom is not strictly positive

- #### homoscedasticTTest

      protected double homoscedasticTTest(double m1,
                              double m2,
                              double v1,
                              double v2,
                              double n1,
                              double n2)
                                   throws MaxCountExceededException,
                                          NotStrictlyPositiveException

  Computes p-value for 2-sided, 2-sample t-test, under the assumption of equal subpopulation variances.

  The sum of the sample sizes minus 2 is used as degrees of freedom.

  Parameters:
    `m1` - first sample mean
    `m2` - second sample mean
    `v1` - first sample variance
    `v2` - second sample variance
    `n1` - first sample n
    `n2` - second sample n
  Returns:
    p-value
  Throws:
    `MaxCountExceededException` - if an error occurs computing the p-value
    `NotStrictlyPositiveException` - if the estimated degrees of freedom is not strictly positive

- #### checkSignificanceLevel

      private void checkSignificanceLevel(double alpha)
                                   throws OutOfRangeException

  Check significance level.
  Parameters:
    `alpha` - significance level
  Throws:
    `OutOfRangeException` - if the significance level is out of bounds.

- #### checkSampleData

      private void checkSampleData(double[] data)
                            throws NullArgumentException,
                                   NumberIsTooSmallException

  Check sample data.
  Parameters:
    `data` - Sample data.
  Throws:
    `NullArgumentException` - if `data` is `null`.
    `NumberIsTooSmallException` - if there is not enough sample data.

- #### checkSampleData

      private void checkSampleData(StatisticalSummary stat)
                            throws NullArgumentException,
                                   NumberIsTooSmallException

  Check sample data.
  Parameters:
    `stat` - Statistical summary.
  Throws:
    `NullArgumentException` - if `data` is `null`.
    `NumberIsTooSmallException` - if there is not enough sample data.

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