In article <[EMAIL PROTECTED]>,
Neil <[EMAIL PROTECTED]> wrote:
>I was wondering what the standard deviation means exactly?
>I've seen the equation, etc., but I don't really understand
>what st dev is and what it is for.
>I am not a statistician as you can tell...
Even if you were, you might know what it is good for, but
not have any idea, other than that, about the name.
The standard deviation is the square root of the variance,
and variances add for sums of uncorrelated random
variables. A more complicated expression results if they
are correlated.
Sums of large numbers of independent random variables with
variances, each of them negligible in the sum, are
approximately normal. If a random variable X is normal
with mean m and standard deviation s, P((X-m)/s < c)
depends only on c; in all cases, the standard deviation
acts as a scale parameter.
I believe the reason it is called the STANDARD deviation
is that if the probability distribution is concentrated
equally at the two points one standard deviation from
the mean, the first two moments agree with that of the
original distribution; the deviation from the mean to
get this is the standard deviation.
--
This address is for information only. I do not claim that these views
are those of the Statistics Department or of Purdue University.
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907-1399
[EMAIL PROTECTED] Phone: (765)494-6054 FAX: (765)494-0558
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