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## weka.core
Class Statistics

    java.lang.Object
      weka.core.Statistics

All Implemented Interfaces:
  RevisionHandler

    public class Statistics

extends java.lang.Object
implements RevisionHandler

Class implementing some distributions, tests, etc. The code is mostly adapted from the CERN Jet Java libraries: Copyright 2001 University of Waikato Copyright 1999 CERN - European Organization for Nuclear Research. Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. CERN and the University of Waikato make no representations about the suitability of this software for any purpose. It is provided "as is" without expressed or implied warranty.

Version:
  $Revision: 5616 $
Author:
  peter.gedeck@pharma.Novartis.com, wolfgang.hoschek@cern.ch, Eibe Frank (eibe@cs.waikato.ac.nz), Richard Kirkby (rkirkby@cs.waikato.ac.nz)

Constructor Summary
`Statistics()`

Method Summary
`static double` | `binomialStandardError(double p, int n)` Computes standard error for observed values of a binomial random variable.
`static double` | `chiSquaredProbability(double x, double v)` Returns chi-squared probability for given value and degrees of freedom.
`static double` | `errorFunction(double x)` Returns the error function of the normal distribution.
`static double` | `errorFunctionComplemented(double a)` Returns the complementary Error function of the normal distribution.
`static double` | `FProbability(double F, int df1, int df2)` Computes probability of F-ratio.
`static double` | `gamma(double x)` Returns the Gamma function of the argument.
`java.lang.String` | `getRevision()` Returns the revision string.
`static double` | `incompleteBeta(double aa, double bb, double xx)` Returns the Incomplete Beta Function evaluated from zero to `xx`.
`static double` | `incompleteBetaFraction1(double a, double b, double x)` Continued fraction expansion #1 for incomplete beta integral.
`static double` | `incompleteBetaFraction2(double a, double b, double x)` Continued fraction expansion #2 for incomplete beta integral.
`static double` | `incompleteGamma(double a, double x)` Returns the Incomplete Gamma function.
`static double` | `incompleteGammaComplement(double a, double x)` Returns the Complemented Incomplete Gamma function.
`static double` | `lnGamma(double x)` Returns natural logarithm of gamma function.
`static void` | `main(java.lang.String[] ops)` Main method for testing this class.
`static double` | `normalInverse(double y0)` Returns the value, `x`, for which the area under the Normal (Gaussian) probability density function (integrated from minus infinity to `x`) is equal to the argument `y` (assumes mean is zero, variance is one).
`static double` | `normalProbability(double a)` Returns the area under the Normal (Gaussian) probability density function, integrated from minus infinity to `x` (assumes mean is zero, variance is one).
`static double` | `p1evl(double x, double[] coef, int N)` Evaluates the given polynomial of degree `N` at `x`.
`static double` | `polevl(double x, double[] coef, int N)` Evaluates the given polynomial of degree `N` at `x`.
`static double` | `powerSeries(double a, double b, double x)` Power series for incomplete beta integral.
`static double` | `stirlingFormula(double x)` Returns the Gamma function computed by Stirling's formula.

Methods inherited from class java.lang.Object
`equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### Statistics

    public Statistics()

Method Detail

### binomialStandardError

    public static double binomialStandardError(double p,
                                               int n)

  Computes standard error for observed values of a binomial random variable.

  Parameters:
    `p` - the probability of success
    `n` - the size of the sample
  Returns:
    the standard error

### chiSquaredProbability

    public static double chiSquaredProbability(double x,
                                               double v)

  Returns chi-squared probability for given value and degrees of freedom. (The probability that the chi-squared variate will be greater than x for the given degrees of freedom.)

  Parameters:
    `x` - the value
    `v` - the number of degrees of freedom
  Returns:
    the chi-squared probability

### FProbability

    public static double FProbability(double F,
                                      int df1,
                                      int df2)

  Computes probability of F-ratio.

  Parameters:
    `F` - the F-ratio
    `df1` - the first number of degrees of freedom
    `df2` - the second number of degrees of freedom
  Returns:
    the probability of the F-ratio.

### normalProbability

    public static double normalProbability(double a)

  Returns the area under the Normal (Gaussian) probability density function, integrated from minus infinity to `x` (assumes mean is zero, variance is one).

                                  x
                                   -
                         1        | |          2
        normal(x)  = ---------    |    exp( - t /2 ) dt
                     sqrt(2pi)  | |
                                 -
                                -inf.

                   =  ( 1 + erf(z) ) / 2
                   =  erfc(z) / 2

  where `z = x/sqrt(2)`. Computation is via the functions `errorFunction` and `errorFunctionComplement`.

  Parameters:
    `a` - the z-value
  Returns:
    the probability of the z value according to the normal pdf

### normalInverse

    public static double normalInverse(double y0)

  Returns the value, `x`, for which the area under the Normal (Gaussian) probability density function (integrated from minus infinity to `x`) is equal to the argument `y` (assumes mean is zero, variance is one).

  For small arguments `0 < y < exp(-2)`, the program computes `z = sqrt( -2.0 * log(y) )`; then the approximation is `x = z - log(z)/z - (1/z) P(1/z) / Q(1/z)`. There are two rational functions P/Q, one for `0 < y < exp(-32)` and the other for `y` up to `exp(-2)`. For larger arguments, `w = y - 0.5`, and `x/sqrt(2pi) = w + w**3 R(w**2)/S(w**2))`.

  Parameters:
    `y0` - the area under the normal pdf
  Returns:
    the z-value

### lnGamma

    public static double lnGamma(double x)

  Returns natural logarithm of gamma function.

  Parameters:
    `x` - the value
  Returns:
    natural logarithm of gamma function

### errorFunction

    public static double errorFunction(double x)

  Returns the error function of the normal distribution. The integral is

                                 x
                                  -
                       2         | |          2
         erf(x)  =  --------     |    exp( - t  ) dt.
                    sqrt(pi)   | |
                                -
                                 0

  Implementation: For `0 <= |x| < 1, erf(x) = x * P4(x**2)/Q5(x**2)`; otherwise `erf(x) = 1 - erfc(x)`.

  Code adapted from the Java 2D Graph Package 2.4, which in turn is a port from the Cephes 2.2 Math Library (C).

  Parameters:
    `a` - the argument to the function.

### errorFunctionComplemented

    public static double errorFunctionComplemented(double a)

  Returns the complementary Error function of the normal distribution.

        1 - erf(x) =

                                 inf.
                                   -
                        2         | |          2
         erfc(x)  =  --------     |    exp( - t  ) dt
                     sqrt(pi)   | |
                                 -
                                  x

  Implementation: For small x, `erfc(x) = 1 - erf(x)`; otherwise rational approximations are computed.

  Code adapted from the Java 2D Graph Package 2.4, which in turn is a port from the Cephes 2.2 Math Library (C).

  Parameters:
    `a` - the argument to the function.

### p1evl

    public static double p1evl(double x,
                               double[] coef,
                               int N)

  Evaluates the given polynomial of degree `N` at `x`. Evaluates polynomial when coefficient of N is 1.0. Otherwise same as `polevl()`.

                           2          N
       y  =  C  + C x + C x  +...+ C x
              0    1     2          N

       Coefficients are stored in reverse order:

       coef[0] = C  , ..., coef[N] = C  .
                  N                   0

  The function `p1evl()` assumes that `coef[N] = 1.0` and is omitted from the array. Its calling arguments are otherwise the same as `polevl()`.

  In the interest of speed, there are no checks for out of bounds arithmetic.

  Parameters:
    `x` - argument to the polynomial.
    `coef` - the coefficients of the polynomial.
    `N` - the degree of the polynomial.

### polevl

    public static double polevl(double x,
                                double[] coef,
                                int N)

  Evaluates the given polynomial of degree `N` at `x`.

                           2          N
       y  =  C  + C x + C x  +...+ C x
              0    1     2          N

       Coefficients are stored in reverse order:

       coef[0] = C  , ..., coef[N] = C  .
                  N                   0

  In the interest of speed, there are no checks for out of bounds arithmetic.

  Parameters:
    `x` - argument to the polynomial.
    `coef` - the coefficients of the polynomial.
    `N` - the degree of the polynomial.

### incompleteGamma

    public static double incompleteGamma(double a,
                                         double x)

  Returns the Incomplete Gamma function.

  Parameters:
    `a` - the parameter of the gamma distribution.
    `x` - the integration end point.

### incompleteGammaComplement

    public static double incompleteGammaComplement(double a,
                                                   double x)

  Returns the Complemented Incomplete Gamma function.

  Parameters:
    `a` - the parameter of the gamma distribution.
    `x` - the integration start point.

### gamma

    public static double gamma(double x)

  Returns the Gamma function of the argument.

### stirlingFormula

    public static double stirlingFormula(double x)

  Returns the Gamma function computed by Stirling's formula. The polynomial STIR is valid for 33 <= x <= 172.

### incompleteBeta

    public static double incompleteBeta(double aa,
                                        double bb,
                                        double xx)

  Returns the Incomplete Beta Function evaluated from zero to `xx`.

  Parameters:
    `aa` - the alpha parameter of the beta distribution.
    `bb` - the beta parameter of the beta distribution.
    `xx` - the integration end point.

### incompleteBetaFraction1

    public static double incompleteBetaFraction1(double a,
                                                 double b,
                                                 double x)

  Continued fraction expansion #1 for incomplete beta integral.

### incompleteBetaFraction2

    public static double incompleteBetaFraction2(double a,
                                                 double b,
                                                 double x)

  Continued fraction expansion #2 for incomplete beta integral.

### powerSeries

    public static double powerSeries(double a,
                                     double b,
                                     double x)

  Power series for incomplete beta integral. Use when b*x is small and x not too close to 1.

### getRevision

    public java.lang.String getRevision()

  Returns the revision string.

  Specified by:
    `getRevision` in interface `RevisionHandler`
  Returns:
    the revision

### main

    public static void main(java.lang.String[] ops)

  Main method for testing this class.

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