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/*
* File: WilcoxonSignedRankConfidence.java
* Authors: Kevin R. Dixon
* Company: Sandia National Laboratories
* Project: Cognitive Foundry
*
* Copyright August 20, 2007, Sandia Corporation. Under the terms of Contract
* DE-AC04-94AL85000, there is a non-exclusive license for use of this work by
* or on behalf of the U.S. Government. Export of this program may require a
* license from the United States Government. See CopyrightHistory.txt for
* complete details.
*
*/
package gov.sandia.cognition.statistics.method;
import gov.sandia.cognition.annotation.PublicationReference;
import gov.sandia.cognition.annotation.PublicationType;
import gov.sandia.cognition.statistics.distribution.UnivariateGaussian;
import gov.sandia.cognition.util.AbstractCloneableSerializable;
import gov.sandia.cognition.util.ArrayIndexSorter;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
/**
* This is a Wilcoxon Signed-Rank Sum test, which performs a pair-wise test
* to determine if two datasets are different.
*
* @author Kevin R. Dixon
* @since 2.0
*
*/
@ConfidenceTestAssumptions(
name="Wilcoxon Signed-Rank Sum Test",
alsoKnownAs="Wilcoxon signed-rank test",
description={
"A nonparameteric statistical hypothesis test for the case of two related samples or repeated measurements on a single sample.",
"Wilcoxon Signed-Rank Sum Test does not assume that the data are generated according to a particular distribution.",
"Used as an alternative to a paired Student-t test when the data are not assumed to be Gaussian."
},
assumptions={
"The differences between paired samples are independent.",
"Each difference comes from a continuous population, identical for all differences, and is symmetric about a common median.",
"The data are oridinal variables such that comparisons such as greater than, less than, and equal to have meaning."
},
nullHypothesis="The median difference between the paired samples is 0.0",
dataPaired=true,
dataSameSize=true,
distribution=UnivariateGaussian.CDF.class,
reference=@PublicationReference(
author="Wikipedia",
title="Wilcoxon signed-rank test",
type=PublicationType.WebPage,
year=2009,
url="http://en.wikipedia.org/wiki/Wilcoxon_signed-rank_test"
)
)
public class WilcoxonSignedRankConfidence
extends AbstractCloneableSerializable
implements NullHypothesisEvaluator>
{
/**
* Default instance since the class has no state.
*/
public static final WilcoxonSignedRankConfidence INSTANCE =
new WilcoxonSignedRankConfidence();
/** Creates a new instance of WilcoxonSignedRankConfidence */
public WilcoxonSignedRankConfidence()
{
}
@Override
public WilcoxonSignedRankConfidence.Statistic evaluateNullHypothesis(
Collection data1,
Collection data2)
{
if( data1.size() != data2.size() )
{
throw new IllegalArgumentException(
"Data1 and data2 must be same size!" );
}
// Find the differences and then rank them
int N = data1.size();
int numNonzero = 0;
ArrayList rawDifference = new ArrayList(N);
ArrayList absDifference = new ArrayList(N);
Iterator i1 = data1.iterator();
Iterator i2 = data2.iterator();
while( i1.hasNext() )
{
// Only add nonzero differences
double difference = i1.next().doubleValue() - i2.next().doubleValue();
if( difference != 0.0 )
{
numNonzero++;
rawDifference.add( difference );
absDifference.add( Math.abs(difference) );
}
}
double[] ranks = WilcoxonSignedRankConfidence.ranks( absDifference );
double positiveRankSum = 0.0;
double negativeRankSum = 0.0;
for( int i = 0; i < numNonzero; i++ )
{
if( rawDifference.get(i) > 0.0 )
{
positiveRankSum += ranks[i];
}
else
{
negativeRankSum += ranks[i];
}
}
double T = Math.min( positiveRankSum, negativeRankSum );
return new WilcoxonSignedRankConfidence.Statistic( T, numNonzero );
}
/**
* Returns the ranks of the values in ascending order
* @param values Collection of values to rank
* @return Ranks of the indices of the values
*/
public static double[] ranks(
Collection values )
{
// Sort them first
int N = values.size();
double[] array = new double[ N ];
int index = 0;
for( Number value : values )
{
array[index] = value.doubleValue();
index++;
}
int[] sortedIndices = ArrayIndexSorter.sortArrayAscending( array );
double[] ranks = new double[ N ];
for( int rank = 1; rank <= N; rank++ )
{
// See if we have any ties
if( (rank < N) &&
(array[sortedIndices[rank-1]] == array[sortedIndices[rank]]) )
{
double sum = rank + (rank+1);
int numEqual = 2;
for( int j = rank+2; j <= N; j++ )
{
if( array[sortedIndices[rank-1]] == array[sortedIndices[j-1]] )
{
sum += j;
numEqual++;
}
else
{
break;
}
}
// Spread the total rank across the tying ranks
for( int i = 0; i < numEqual; i++ )
{
ranks[sortedIndices[rank+i-1]] = sum/numEqual;
}
rank += numEqual-1;
}
else
{
ranks[sortedIndices[rank-1]] = rank;
}
}
return ranks;
}
/**
* ConfidenceStatistics associated with a Wilcoxon test
*/
public static class Statistic
extends AbstractConfidenceStatistic
{
/**
* Wilcoxon T statistic, minimum sign-rank sum
*/
private double T;
/**
* Number of nonzero differences in the data
*/
private int numNonZero;
/**
* Z-statistic for the Gaussian CDF
*/
private double z;
/**
* Creates a new instance of Statistic
* @param T
* Wilcoxon T statistic, minimum sign-rank sum
* @param numNonZero
* Number of nonzero differences in the data
*/
public Statistic(
double T,
int numNonZero )
{
this( T, numNonZero, computeZ(T,numNonZero) );
}
/**
* Creates a new instance of Statistic
* @param T
* Wilcoxon T statistic, minimum sign-rank sum
* @param numNonZero
* Number of nonzero differences in the data
* @param z
* Z-statistic for the Gaussian CDF
*/
private Statistic(
double T,
int numNonZero,
double z )
{
super( computeNullHypothesisProbability( z ) );
this.setT( T );
this.setNumNonZero( numNonZero );
this.setZ( z );
}
/**
* Computes the z-value from the T-statistic and numNonZero value
* @param T
* Wilcoxon T statistic, minimum sign-rank sum
* @param numNonZero
* Number of nonzero differences in the data
* @return
* Z-statistic for the Gaussian CDF
*/
protected static double computeZ(
double T,
int numNonZero )
{
double numerator = T - numNonZero*(numNonZero+1)/4.0;
double denominator = Math.sqrt( numNonZero*(numNonZero+1)*(2*numNonZero+1)/24.0 );
double z = numerator / denominator;
return z;
}
/**
* Computes the p-value given the z-value
* @param z
* Z-statistic for the Gaussian CDF
* @return
* p-value for this Wilcoxon Test
*/
protected static double computeNullHypothesisProbability(
double z )
{
return 2.0*UnivariateGaussian.CDF.evaluate( -Math.abs(z), 0, 1 );
}
/**
* Getter for T
* @return
* Wilcoxon T statistic, minimum sign-rank sum
*/
public double getT()
{
return this.T;
}
/**
* Getter for T
* @param T
* Wilcoxon T statistic, minimum sign-rank sum
*/
protected void setT(
double T)
{
this.T = T;
}
/**
* Getter fir numNonZero
* @return
* Number of nonzero differences in the data
*/
public int getNumNonZero()
{
return this.numNonZero;
}
/**
* Setter for numNonZero
* @param numNonZero
* Number of nonzero differences in the data
*/
protected void setNumNonZero(
int numNonZero)
{
this.numNonZero = numNonZero;
}
/**
* Getter for z
* @return
* Z-statistic for the Gaussian CDF
*/
public double getZ()
{
return this.z;
}
/**
* Setter for z
* @param z
* Z-statistic for the Gaussian CDF
*/
protected void setZ(
double z)
{
this.z = z;
}
@Override
public double getTestStatistic()
{
return this.getZ();
}
}
}