// =================================================================================================
// Copyright 2011 Twitter, Inc.
// -------------------------------------------------------------------------------------------------
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this work except in compliance with the License.
// You may obtain a copy of the License in the LICENSE file, or 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.twitter.common.stats;
/**
* Calculate significance scores between an observed amount and an expected amount.
*
* @author Gilad Mishne
*/
public class Significance {
/**
* @param observed The observed amount.
* @param expected The expected amount.
* @return [(observed - expected) ** 2 / expected] * sign(observed - expected)
*/
public static double chiSqrScore(double observed, double expected) {
double score = Math.pow((observed - expected), 2) / expected;
if (observed < expected) {
score *= -1;
}
return score;
}
/**
* @param observed The observed amount.
* @param expected The expected amount.
* @return -2 * expected * log(observed / expected) * sign(observed - expected)
*/
public static double logLikelihood(double observed, double expected) {
if (observed == 0) {
return -expected;
}
if (expected == 0) {
return observed;
}
double score = -2 * observed * Math.log(observed / expected);
if (observed < expected) {
score *= -1;
}
return score;
}
private Significance() {
// prevent instantiation
}
}