[rrd-users] percentile calculations

Dan Cech dcech at phpwerx.net
Tue Sep 5 20:01:03 MEST 2006

Hello All,

I have a question regarding calculating percentiles.

Specifically, I have a graph showing data over which I need to overlay a
95th percentile line.

However, when I'm displaying the graph for the current month, the
PERCENT function is using all the unknown future values in the
calculation, causing it to be incorrect.

As a very simplified example, say I'm 10 days into a month (with 20 days
remaining) and the values so far look like this:


The 90th percentile should be 9, but in fact it will be 7 because
PERCENT will use the 20 NaN values in the calculation.

I have looked through the documentation and can't find any mechanism
which would allow me to restrict the PERCENT function to a specific date
range (to exclude values in the future), or exclude NaN values.

Any suggestions would be appreciated.



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