[MATH-691] Statistics.setVarianceImpl makes getStandardDeviation produce NaN

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 2.2
• Fix Version/s: 3.0

Environment

Windows 7 64-bit, java version 1.6.0_23

• Assignee: Phil Steitz
• Reporter: Warren Tang
• Created: 16/Oct/11 17:18
• Updated: 24/Mar/12 16:16
• Resolved: 27/Nov/11 05:20

Description

Invoking SummaryStatistics.setVarianceImpl(new Variance(true/false) makes getStandardDeviation produce NaN. The code to reproduce it:

int[] scores = {1, 2, 3, 4};
SummaryStatistics stats = new SummaryStatistics();
stats.setVarianceImpl(new Variance(false)); //use "population variance"
for(int i : scores) {
  stats.addValue(i);
}
double sd = stats.getStandardDeviation();
System.out.println(sd);

A workaround suggested by Mikkel is:

  double sd = FastMath.sqrt(stats.getSecondMoment() / stats.getN());