On Tue, Oct 22, 2013 at 11:16:19AM -0700, Jeff Janes wrote:
> On Mon, Oct 21, 2013 at 4:29 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Hm. It's been a long time since college statistics, but doesn't the
> > entire concept of standard deviation depend on the assumption that the
> > underlying distribution is more-or-less normal (Gaussian)?
>
> It is easy to misinterpret the standard deviation if the distribution is
> not gaussian, but that is also true of the average. The standard deviation
> (or the variance) is commonly used with non-gaussian distributions, either
> because it is the most efficient estimator for those particular
> distributions, or just because it is so commonly available.
Well, the standard deviation is the square root of the variance, which
is the second moment of the distribution. The first moment being the
mean. No matter what distribution it is, these are useful numbers.
If I had to guess a distribution for query runtimes I'd go for Poisson,
which would mean you'd expect the mean to equal the variance. Don't
have enough experience with such measurements to say whether that is
reasonable.
Have a nice day,
--
Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
> He who writes carelessly confesses thereby at the very outset that he does
> not attach much importance to his own thoughts. -- Arthur Schopenhauer