[rrd-users] Statistical Weirdness

Joe Loiacono jloiacon at csc.com
Thu Jan 17 21:32:01 CET 2008

I recently began creating a fifth graph (Last 3 years) to go along with
the typical MRTG-like four (Last Day, Week, Month, Year). But, I noticed
that the statistics for the new 3 Year graph do not match the 1 year graph
even though there are only about 7 months worth of data, and you would
think they should.

I manually checked the values calculated for the 1 year graph (365 days)
and they are correct. Things go a little whacked when I start the graphs
601 days prior to the present. All statistical values make a quantum
change, and from that point back in time, only the 95th percentile
changes, and then at a period of every 40 days. E.g., How can the MAXIMUM
change?

Also note that the 95th percentile seems to take 'nan's into account even
in the 'good' period, whereas the other statistics don't.

These are the results I got for various graphs:

Start Days    MAX.     95th.    AVG.     MIN.

-365          477.76   333.53   235.46   000.00
-430          477.76   328.84   235.46   000.00
-530          477.76   324.09   235.46   000.00
-600          477.76   316.29   235.46   000.00
-601          422.70   296.95   236.37   102.70 *** ALL CHANGE ***
-602          422.70   294.30   236.37   102.00 *** 95 Pct. STEP CHANGE
***
-605          422.70   294.30   236.37   102.00
-610          422.70   294.30   236.37   102.00
-615          422.70   294.30   236.37   102.00
-630          422.70   294.30   236.37   102.00
-635          422.70   294.30   236.37   102.00
-640          422.70   294.30   236.37   102.00
-641          422.70   294.30   236.37   102.00
-642          422.70   294.26   236.37   102.00 *** 95 Pct. STEP CHANGE
***
-645          422.70   294.26   236.37   102.00
-660          422.70   294.26   236.37   102.00
-675          422.70   294.26   236.37   102.00
-680          422.70   294.26   236.37   102.00
-681          422.70   294.26   236.37   102.00
-682          422.70   290.63   236.37   102.00 *** 95 Pct. STEP CHANGE
***
-685          422.70   290.63   236.37   102.00
-690          422.70   290.63   236.37   102.00
-710          422.70   290.63   236.37   102.00
-720          422.70   290.63   236.37   102.00
-721          422.70   290.63   236.37   102.00
-722          422.70   285.31   236.37   102.00 *** 95 Pct. STEP CHANGE
***
-725          422.70   285.31   236.37   102.00
-730          422.70   285.31   236.37   102.00

Joe
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