http://lucene.apache.org/lucene-parent/lucene-core: Apache Lucene Java Core (The Apache Software Foundation) 
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package org.apache.lucene.search.similarities;



Normalized chi-squared measure of distance from independence


Described as:
"can be used for tasks that require high precision, against both 
 short and long queries."

@lucene.experimental
/**
 * Normalized chi-squared measure of distance from independence
 * <p>
 * Described as:
 * "can be used for tasks that require high precision, against both 
 *  short and long queries."
 * @lucene.experimental
 */
public class IndependenceChiSquared extends Independence {
  
  
Sole constructor.

/**
   * Sole constructor.
   */
  public IndependenceChiSquared() {}

  @Override
  public double score(double freq, double expected) {
    return (freq - expected) * (freq - expected) / expected;
  }

  @Override
  public String toString() {
    return "ChiSquared";
  }
}