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https://issues.apache.org/jira/browse/MATH-1185?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14264477#comment-14264477
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Luc Maisonobe commented on MATH-1185:
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Yes, there is a reason: it is due to the subtraction, not the the distribution.
When you compute 1.0 - x, you are limited with IEEE754 double precision. If x
is close to 1.0, the subtraction
leads to loss of significance (also called catastrophic cancellation). What you
see here is that the last
number just below 1.0 is 1.0 - 2^-53. In other words, there are no
representable number between 1.0-2^-53 and 1.0.
Then, when subtraction is computed, you jump from 2^-53 to 0.
> Tail probability drops to zero beyond 10e-17 ?
> ----------------------------------------------
>
> Key: MATH-1185
> URL: https://issues.apache.org/jira/browse/MATH-1185
> Project: Commons Math
> Issue Type: Wish
> Affects Versions: 3.3, 3.4
> Reporter: Sriram Natarajan
> Priority: Minor
>
> This could be a simple question, In which case I can expect a clarification.
> If this is the wrong place to post such a question, let me know.
> OS: Windows 8.1, Java 1.8.0_25
> ChiSquaredDistribution chisq = new ChiSquaredDistribution(23)
> 1.0 - chisq.cumulativeProbability(130) is 1.1102230246251565E-16
> 1.0 - chisq.cumulativeProbability(131) is 0.0
> Am pretty sure it is not a formatting issue. Is there a reason why the tail
> drops to zero at this point?
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