[rrd-users] Re: Median calculations ?

Serge Maandag serge.maandag at staff.zeelandnet.nl
Tue Mar 14 20:31:35 MET 2006

> My apologies :) so the 50th percentile would be {2,3,4} here? 
> (or without the 2...)


I haven't ever used the percentile function of rrdtool, but it should
just return 2.

In your example the 20th percentile is 1, the 40th 2 etc. etc. and the 
100th percentile is 50.

The rrdtool documentation says:

This should follow a DEF or CDEF vname. The vname is popped, another 
number is popped which is a certain percentage (0..100). The data set 
is then sorted and the value returned is chosen such that percentage 
percent of the values is lower or equal than the result. Unknown values 
are considered lower than any finite number for this purpose so if this 
operator returns an unknown you have quite a lot of them in your data. 
Infinite numbers are lesser, or more, than the finite numbers and are 
always more than the Unknown numbers. (NaN < -INF < finite values < INF)
Example: VDEF:perc95=mydata,95,PERCENT

I don't think it will interpolate if there's an even number of elements
in the array.


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