commit 97d23965e8c8770b2b52263b3576681ece4c43a0
author aherbert <aherbert@apache.org> Wed Sep 11 17:10:26 2019 +0100
committer aherbert <aherbert@apache.org> Wed Sep 11 17:10:26 2019 +0100
tree ce77fa0ce9e8b0f9bc62bb18afa04b57eba2b9bc
parent 7977dfeabc46275afbf99d6dfd24723d095e35ab

LogisticsDistribution: Fix the variance computation

commons-statistics-distribution/src/main/java/org/apache/commons/statistics/distribution/LogisticDistribution.java

diff --git a/commons-statistics-distribution/src/main/java/org/apache/commons/statistics/distribution/LogisticDistribution.java b/commons-statistics-distribution/src/main/java/org/apache/commons/statistics/distribution/LogisticDistribution.java
index ecf9efb..985e289 100644
--- a/commons-statistics-distribution/src/main/java/org/apache/commons/statistics/distribution/LogisticDistribution.java
+++ b/commons-statistics-distribution/src/main/java/org/apache/commons/statistics/distribution/LogisticDistribution.java
@@ -105,7 +105,7 @@
     /** {@inheritDoc} */
     @Override
     public double getVariance() {
-        return oneOverScale * oneOverScale * PI_SQUARED_OVER_THREE;
+        return scale * scale * PI_SQUARED_OVER_THREE;
     }
 
     /** {@inheritDoc} */

commons-statistics-distribution/src/test/java/org/apache/commons/statistics/distribution/LogisticsDistributionTest.java

diff --git a/commons-statistics-distribution/src/test/java/org/apache/commons/statistics/distribution/LogisticsDistributionTest.java b/commons-statistics-distribution/src/test/java/org/apache/commons/statistics/distribution/LogisticsDistributionTest.java
index 1aa6d0e..310d8d8 100644
--- a/commons-statistics-distribution/src/test/java/org/apache/commons/statistics/distribution/LogisticsDistributionTest.java
+++ b/commons-statistics-distribution/src/test/java/org/apache/commons/statistics/distribution/LogisticsDistributionTest.java
@@ -40,6 +40,16 @@
         Assertions.assertTrue(d.isSupportConnected());
     }
 
+    @Test
+    public void testMeanAndVariance() {
+        LogisticDistribution d = makeDistribution();
+        // Constructor 'location' parameter = mean
+        Assertions.assertEquals(2.0, d.getMean());
+        // Variance = (s^2 * pi^2) / 3
+        // Constructor 'scale' parameter = s
+        Assertions.assertEquals(5 * 5 * Math.PI * Math.PI / 3, d.getVariance());
+    }
+
     @Override
     public LogisticDistribution makeDistribution() {
         return new LogisticDistribution(2, 5);