Hi all, On Aug 19, 9:48 am, [email protected] (Tobias Oetiker) wrote: > Hoi Joshua, > > Thursday Joshua Keroes wrote: > > How is percentile calculated? We've been calculating this manually, since > > before rrdtool offered a PERCENT function. We discovered that our manual > > calculation and the one supplied by rrdtool differ by one index. For example > > (percentile.pl program attached): > > > [Thu Aug 13, 19:58:41 | 692] $ percentile.pl -v --cf=AVERAGE -- ds=ingress > > --dsi=0 --start=1242864000 --end=1245542399 etkfgl624456ubielg01g.rrd > > > percentile.pl using RRDs-1.2019 > > etkfgl624456ubielg01g.rrd: > > main::ptile_fetch() > > start = 05/21/09 > > end = 06/20/09 > > rows = 31 > > 95% of 31 = 29.45 > > 95th %-ile row index = 29 > > discarded rows of -NaN's = 0 > > row 28 = *1126767.40* > > row 29 = *1521699.32* > > row 30 = 4132277.36 > > manual calculation using fetch() = *1521699.32* > > PERCENT calculation using graph() = *1126767.40* > > difference ~ 394931 (26%) > > > Our manual calculation is grabbing row 29 and PERCENT is grabbing row 28. > > 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? Thanks, Leander _______________________________________________ rrd-users mailing list [email protected] https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users
