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Date: Fri, 10 Jun 2011 17:51:59 +0000 (UTC)
From: "Christopher Nix (JIRA)" <jira@apache.org>
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Subject: [jira] [Issue Comment Edited] (MATH-582) Percentile does not work
 as described in API
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    [ https://issues.apache.org/jira/browse/MATH-582?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13047318#comment-13047318 ] 

Christopher Nix edited comment on MATH-582 at 6/10/11 5:51 PM:
---------------------------------------------------------------

Added a patch for org.apache.commons.math.stat.Percentile documentation in order to better describe the correct implementation of percentiles as recommended by NIST.

      was (Author: joubert):
    Added a patch for org.apache.commons.math.stat.Percentile documentation in order to better describe the correct implementation percentiles as recommended by NIST.
  
> Percentile does not work as described in API
> --------------------------------------------
>
>                 Key: MATH-582
>                 URL: https://issues.apache.org/jira/browse/MATH-582
>             Project: Commons Math
>          Issue Type: Bug
>    Affects Versions: 2.2
>            Reporter: Andre Herbst
>         Attachments: MATH-582.patch
>
>
> example call:
> StatUtils.percentile(new double[]{0d, 1d}, 25)   returns 0.0
> The API says that there is a position being computed:  p*(n+1)/100 -> we have p=25 and n=2
> I would expect position 0.75 as result. Next step according to the API is: interpolation between both values at floor(0.25) and at ceil(0.25). Those values are 0d and 1d ... so lower + d * (upper - lower) should give 0d + 0.25*(1d - 0d) = 0.25
> But the above call returns 0 as result. This does not make sense to me.
> another example where I think the result is not correct:
> StatUtils.percentile(new double[]{0d, 1d, 1d, 1d}, 25)   returns 0.25
> we have pos = 25*5/100 = 1.25  ... so d = 0.25
> values at position floor(1.25) and ceil(1.25) are 1d and 1d. How comes that the result is not between 1d?

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