[rrd-users] Re: percentile calculations

[Alex van den Bogaerdt]

On Tue, Sep 05, 2006 at 03:05:24PM -0400, Dan Cech wrote:

> >>              causing it to be incorrect.
> > 
> > Why?
> 
> In this case I don't mean incorrect from a mathematical standpoint, but
> in terms of the purpose of the report.

[snip further explanation]

OK.

Does this help:

snippet from rrdgraph.pod:

 Data and variables
           DEF:vname=rrdfile:ds-name:CF[:step=step][:start=time][:end=time]


> >> 1,2,3,4,5,6,7,8,9,10
> >>
> >> The 90th percentile should be 9
> > 
> > according to what/who ?
> 
> The 90th percentile of the values above is 9.

Yes.  But you are not asking for the 90 percentile of these values.


> I understand that the PERCENT function will include the 20 UNKN values
> and produce the answer 7 which is also mathematically correct but
> 'incorrect' from the point of view of this particular application.

Most of the times, computers do what you ask them to do, not what you
want them to do.  This does seem to be a typical case.

Please try that start and end time def.  I haven't been playing with
it for a while but if memory serves me well it does have some quircks.
More specifically, if you have another DEF accessing the same data,
things go wrong. This would be the exception to the rule I mentioned
two paragraphs above.


HTH
-- 
Alex van den Bogaerdt
http://www.vandenbogaerdt.nl/rrdtool/

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