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From "Andre Herbst (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MATH-582) Percentile does not work as described in API
Date Tue, 24 May 2011 11:34:47 GMT
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


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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