Hi Nick,
I cannot tell you how ashamed I am. This should've been absolutely
obvious to me (I also did graduate studies in Statistics, after which I
switched to Economics, and Finance.) I guess it's TGIF feeling, or just
workload, but my mind didn't perhaps work, when I failed to find this
simple reason!
Thanks very much,
-tir
On Fri, 15 Mar 2002 [EMAIL PROTECTED] wrote:
>
> Hi Tirthankar,
>
> I knew my Graduate work in Statistics would come in handy at some point...
> :-) Without looking at the documentation for Math:NumberCruncher, I can
> fairly confidently say that both you and Math:NumberCruncher are right, but
> you are trying to calculate different things. In the following
> explanation, I'll use the statistical language loosely, because you'll want
> to consult a statistics textbook (or possibly the documentation for
> Math:NumberCruncher) for the details.
>
> If the numbers 0..20 represent a sample of 21 independent random variables
> from a given population (or distribution), the estimated variance is indeed
> 38.5. Keep in mind that this is an estimate of the true variance, which
> presumably you don't know because you just have a sample of all possible
> values.
>
> If the numbers 0..20 represent every possible value in the population, and
> each value is equally likely, then the actual variance - which can be
> calculated because you know each value and its probability - is 36.6666666,
> and this is the value returned by Math:NumberCruncher.
>
> So one answer is an (unbiased) estimate of the true variance and the other
> is a calculation of the true variance, and which one you use depends on
> whether you have a sample or the entire population. In most cases (where
> samples are involved) you'll want to use the first calculation.
>
> This relationship is true: (n-1)/n*(sample variance estimate) = (population
> variance calculation) so that
>
> (20/21)* 38.5 = 36.66667
>
> Hope this helps,
> -Nick
>
>
>
>
> "Tirthankar
>
> C. Patnaik" To: [EMAIL PROTECTED]
>
> <[EMAIL PROTECTED] cc:
>
> .in> Subject: Trouble with
>Math::NumberCruncher
>
>
> 03/15/02
>
> 06:23 AM
>
>
>
>
>
>
>
>
> This might seem trivial, but it has me stumped.
>
> The variance of numbers 0..20 is 38.5.
> Then why does Math:NumberCruncher show it as 36.6667?
>
> Here's a sample prog, and it's output. Here test04.dat is a file that has
> numbers 0 to 20, on separate lines.
> #-------------------start----------------
> #!/usr/bin/perl -w
> use Math::NumberCruncher;
> # Program to test Math::NumberCruncher
>
> # Get a file from STDIN, print the variance, and the std. deviation.
>
> @array = <>;
> map {chomp} @array;
>
> $variance = Math::NumberCruncher::Variance(\@array);
> $stddev = Math::NumberCruncher::StandardDeviation(\@array);
> $sd2 = $stddev**2;
>
> print scalar(@array), "\n";
>
> printf "Var=$variance, S.D=$stddev S.D^2 = $sd2\n";
>
> exit;
> #----------------end---------------------
>
> And the output is:
>
> 21
> Var=36.6666666666667, S.D=6.05530070819498 S.D^2 = 36.6666666666667
>
>
> What could be possibly wrong? Even the standard value is wrong.
>
>
> TIA,
> -tir
>
>
> --
> Tirthankar, IGIDR.
> +91-22-8400919 x275 (r), x593(o), x542(CFL).
> http://www.igidr.ac.in/~tir
>
> LEGACY, n. A gift from one who is legging it out of this vale of
> tears.
>
>
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>
>
>
>
--
Tirthankar, IGIDR.
+91-22-8400919 x275 (r), x593(o), x542(CFL).
http://www.igidr.ac.in/~tir
NON-COMBATANT, n. A dead Quaker.
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