# [rrd-users] Re: Median calculations ?

Alex Prinsier aphex at aphexer.dyndns.org
Wed Mar 15 09:41:01 MET 2006

```I see I made a mistake in my previous mail, the median should be 3 of
course not 2 (forgot to sort ;)).

How exactly do you define the nth percentile? When you have an uneven
number of elements and you take the 50th percentile, do you leave of
(n-1)/2 elements or the (n+1)/2 elements? I believe that you leave of
(n-1)/2 elements in rrdtool's implementation. I'm not sure which one is
more correct...

So the relation with the median: if there are an uneven number of
elements you give the array[(n-1)/2] element which is the median.

If there are an even number of elements you also give array[(n-1)/2] but
the median would be the average between array[(n-1)/2] and array[(n-1)/2].

I'd say that for a median you can use the 50th percentile, it's a good
approximation :)

Alex

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