com.itemanalysis.psychometrics.polycor.PearsonCorrelation.java Source code

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Here is the source code for com.itemanalysis.psychometrics.polycor.PearsonCorrelation.java

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/*
 * Copyright 2012 J. Patrick Meyer
 *
 * Licensed 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
 *
 *     http://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.
 */
package com.itemanalysis.psychometrics.polycor;

import org.apache.commons.math3.stat.descriptive.moment.StandardDeviation;

/**
 * This class is mainly a short-cut for the Pearson correlation.
 * The same computations are provided in Covariance.java with the exception
 * of the correctedValue() method. This method is only found here and it
 * is used primarily for correcting a correlation for spuriousness
 * for an item analysis.
 *
 *
 * @author J. Patrick Meyer
 */
public class PearsonCorrelation implements Comparable<PearsonCorrelation> {

    private Covariance covariance = null;

    private StandardDeviation sdX = null;

    private StandardDeviation sdY = null;

    public PearsonCorrelation() {
        covariance = new Covariance();
        sdX = new StandardDeviation();
        sdY = new StandardDeviation();
    }

    public void increment(Double X, Double Y) {
        if (X != null || Y != null) {
            covariance.increment(X, Y);
            sdX.increment(X);
            sdY.increment(Y);
        }
    }

    /**
     * Correct correlation for spuriousness. This method assumes that
     * the test item is Y and the test score is X. Used for the
      * point-biserial and biserial correlation in an item analysis.
     *
     * @return correlation corrected for spuriousness
     */
    public Double correctedValue() {
        double testSd = sdX.getResult();
        double itemSd = sdY.getResult();
        double rOld = this.value();
        double denom = Math.sqrt(itemSd * itemSd + testSd * testSd - 2 * rOld * itemSd * testSd);
        if (denom == 0.0)
            return Double.NaN;
        return (rOld * testSd - itemSd) / denom;
    }

    public double value() {
        return covariance.correlation().doubleValue();
    }

    public double value(boolean unbiased) {
        return covariance.correlation(unbiased).doubleValue();
    }

    public double sampleSize() {
        return covariance.sampleSize();
    }

    public int compareTo(PearsonCorrelation that) {
        double thisV = this.value();
        double thatV = that.value();
        if (thisV > thatV)
            return 1;
        if (thisV < thatV)
            return -1;
        return 0;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if ((obj == null) || (obj.getClass() != this.getClass()))
            return false;
        Double v = new Double(this.value());
        return ((Double) obj) == v;

    }

    @Override
    public int hashCode() {
        Double v = new Double(this.value());
        return v.hashCode();
    }

}