On Tue, Aug 04, 2009 at 10:45:52AM -0400, Tom Lane wrote: > Sam Mason <s...@samason.me.uk> writes: > > t = 0.54 ((avg1 - avg2) / (stddev * sqrt(2/samples))) > > > We then have to choose how certain we want to be that they're actually > > different, 90% is a reasonably easy level to hit (i.e. one part in ten, > > with 95% being more commonly quoted). For 20 samples we have 19 degrees > > of freedom--giving us a cut-off[1] of 1.328. 0.54 is obviously well > > below this allowing us to say that there's no "statistical significance" > > between the two samples at a 90% level. > > Hmm, so what about 95% or 99% confidence?
The cut-off goes up to 1.729 for 95% and to 2.539 for 99%. These values are only really for a 20 samples with the above calculation, the link I gave above gives a nice table for different values. I've also realized that I did the standard deviation all wrong. I should have calculated them independently and then got the mean: stddev1 = 159.9699 stddev2 = 129.6466 stddev = 144.8083 ((stddev1+stddev2) / 2) Here it makes absolutely no difference, but when they were really different distributions it would. -- Sam http://samason.me.uk/ -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers