org.apache.commons.math3.stat.descriptive.moment

Class Skewness

java.lang.Object

org.apache.commons.math3.stat.descriptive.AbstractUnivariateStatistic

org.apache.commons.math3.stat.descriptive.AbstractStorelessUnivariateStatistic

org.apache.commons.math3.stat.descriptive.moment.Skewness

All Implemented Interfaces:

java.io.Serializable, StorelessUnivariateStatistic, UnivariateStatistic, MathArrays.Function

public class Skewness
extends AbstractStorelessUnivariateStatistic
implements java.io.Serializable

Computes the skewness of the available values.

We use the following (unbiased) formula to define skewness:

skewness = [n / (n -1) (n - 2)] sum[(x_i - mean)^3] / std^3

where n is the number of values, mean is the Mean and std is the StandardDeviation

Note that this statistic is undefined for n < 3. Double.Nan is returned when there is not sufficient data to compute the statistic. Double.NaN may also be returned if the input includes NaN and / or infinite values.

Note that this implementation is not synchronized. If multiple threads access an instance of this class concurrently, and at least one of the threads invokes the increment() or clear() method, it must be synchronized externally.

See Also:

Serialized Form

Constructor Summary

Constructors

Constructor and Description
Skewness() Constructs a Skewness
Skewness(Skewness original) Copy constructor, creates a new Skewness identical to the original
Skewness(org.apache.commons.math3.stat.descriptive.moment.ThirdMoment m3) Constructs a Skewness with an external moment

Method Summary

Modifier and Type	Method and Description
void	clear() Clears the internal state of the Statistic
Skewness	copy() Returns a copy of the statistic with the same internal state.
static void	copy(Skewness source, Skewness dest) Copies source to dest.
double	evaluate(double[] values, int begin, int length) Returns the Skewness of the entries in the specifed portion of the input array.
long	getN() Returns the number of values that have been added.
double	getResult() Returns the value of the statistic based on the values that have been added.
void	increment(double d) Updates the internal state of the statistic to reflect the addition of the new value.

Methods inherited from class org.apache.commons.math3.stat.descriptive.AbstractStorelessUnivariateStatistic

equals, evaluate, hashCode, incrementAll, incrementAll

Methods inherited from class org.apache.commons.math3.stat.descriptive.AbstractUnivariateStatistic

evaluate, getData, setData, setData

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Skewness

public Skewness()

Constructs a Skewness

Skewness

public Skewness(org.apache.commons.math3.stat.descriptive.moment.ThirdMoment m3)

Constructs a Skewness with an external moment

Parameters:

m3 - external moment

Skewness

public Skewness(Skewness original)
         throws NullArgumentException

Copy constructor, creates a new Skewness identical to the original

Parameters:

original - the Skewness instance to copy

Throws:

NullArgumentException - if original is null

Method Detail

increment

public void increment(double d)

Updates the internal state of the statistic to reflect the addition of the new value.

Note that when Skewness(ThirdMoment) is used to create a Skewness, this method does nothing. In that case, the ThirdMoment should be incremented directly.

Specified by:

increment in interface StorelessUnivariateStatistic

Specified by:

increment in class AbstractStorelessUnivariateStatistic

Parameters:

d - the new value.

getResult

public double getResult()

Returns the value of the statistic based on the values that have been added.

See Skewness for the definition used in the computation.

Specified by:

getResult in interface StorelessUnivariateStatistic

Specified by:

getResult in class AbstractStorelessUnivariateStatistic

Returns:

the skewness of the available values.

getN

public long getN()

Returns the number of values that have been added.

Specified by:

getN in interface StorelessUnivariateStatistic

Returns:

the number of values.

clear

public void clear()

Clears the internal state of the Statistic

Specified by:

clear in interface StorelessUnivariateStatistic

Specified by:

clear in class AbstractStorelessUnivariateStatistic

evaluate

public double evaluate(double[] values,
                       int begin,
                       int length)
                throws MathIllegalArgumentException

Returns the Skewness of the entries in the specifed portion of the input array.

See Skewness for the definition used in the computation.

Throws IllegalArgumentException if the array is null.

Specified by:

evaluate in interface UnivariateStatistic

Specified by:

evaluate in interface MathArrays.Function

Overrides:

evaluate in class AbstractStorelessUnivariateStatistic

Parameters:

values - the input array

begin - the index of the first array element to include

length - the number of elements to include

Returns:

the skewness of the values or Double.NaN if length is less than 3

Throws:

MathIllegalArgumentException - if the array is null or the array index parameters are not valid

See Also:

UnivariateStatistic.evaluate(double[], int, int)

copy

public Skewness copy()

Returns a copy of the statistic with the same internal state.

Specified by:

copy in interface StorelessUnivariateStatistic

Specified by:

copy in interface UnivariateStatistic

Specified by:

copy in class AbstractStorelessUnivariateStatistic

Returns:

a copy of the statistic

copy

public static void copy(Skewness source,
                        Skewness dest)
                 throws NullArgumentException

Copies source to dest.

Neither source nor dest can be null.

Parameters:

source - Skewness to copy

dest - Skewness to copy to

Throws:

NullArgumentException - if either source or dest is null