[rrd-users] Percentile off-by-one

[Tobias Oetiker]

Hi Leander,

Today Leander Koornneef wrote:
>  >
>  > hmmm the heart of the problem is, that you have too small a sample
>  > set ... if the difference between picken one row or the next is 26%
>  > you should NOT be using percentile since the results will be more
>  > or less random ...
>
> I'm also experiencing discrepancies in PERCENT calculations as done by
> rrdtool
> when compared to other tools, like the Statistics::Descriptive perl
> module. I've
> also tested with Excel and Numbers (from Apple iWork). The difference
> between
> outcomes appears to increase with larger datasets. Here are the 90th
> percentile
> results from rrdtool and my own script (which uses
> Statistics::Descriptive):
>
> # of days		rrdtool		Statistics::Descriptive
> 1			2.25066		2.26135
> 10			1.37383		1.49129
> 20			1.30663		1.47212
> 30			1.48260		1.67608
>
> This last dataset (30 days) consists of about 9000 values and in this
> dataset,
> the value 1.48260 is actually more like the 87th percentile. Both
> Excel and Numbers
> agree with Statistics::Descriptive here, so I'm inclined to suspect an
> error/bug in
> rrdtool?

you must make sure that the data resolution is the same since this
has a major impact on PERCENT calculation ... best is to set the
step on the DEF line ... also make sure there is actually
sufficient high resolution data in the rrd file ...

cheers
tobi

>
> Thanks,
> Leander
>
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>

-- 
Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900