[MATH-1146] class Mean returns incorrect result after processing an Infinity value

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.3
• Fix Version/s: 3.4
• Assignee: Phil Steitz
• Reporter: david cogen
• Created: 15/Aug/14 18:03
• Updated: 26/Dec/14 19:50
• Resolved: 16/Dec/14 23:53

Description

1. Create a Mean object.
2. call increment() with Double.POSITIVE_INFINITY.
3. Call getResult(). Result is INFINITY as expected.
4. call increment() with 0.
5. Call getResult(). Result is NaN; not INFINITY as expected.

This is apparently due to the "optimization" for calculating mean described in the javadoc. Rather than accumulating a sum, it maintains a running mean value using the formula "m = m + (new value - m) / (number of observations)", which unlike the "definition way", fails after an infinity.

I was using Mean within a SummaryStatistics. Other statistics also seem to be affected; for example, the standard deviation also incorrectly gives NaN rather than Infinity. I don't know if that's due to the error in Mean or if the other stats classes have similar bugs.