Nancy,
I do not have a copy of Aron to hand, but I would be surprised if they
didn't have the sample formula somewhere in the book. Some books do all
the population stuff first, and then all the sample stuff later. In
fact, Aron is right (if a little idiosyncratic) to call the formula with
N in the demoninator "the" formula for the standard deviation. That
formula, in fact, gives one "the" standard deviation of the numbers on
which it is calculated. Now, if one wanted (as we often do) to attempt
to estimate the standard deviation of some theoretical population of
numbers from which this set of numbers was randomly selected, then one
could get a better estimate by using N-1 in the demoninator, but that
number is not (strictly speaking) the standard deviation of the numbers
at hand.
Why not just take this opportunity to explain all this to your students?
Regards,
Chris
--
Christopher D. Green
Department of Psychology
York University
Toronto, ON M3J 1P3
Canada
416-736-5115 ex. 66164
[EMAIL PROTECTED]
http://www.yorku.ca/christo
"All warfare is based on deception."
Sun-tzu, The Art of War, I.18
=============================
[EMAIL PROTECTED] wrote:
Hello,
My whole academic life I have been doing the standard deviation
calculation with a denominator of N-1. Everybook I've used has this as
the formula at least for the sample version (as opposed to the
population version), all my notes and powerpoints make reference to
this, and as far as I can see, SPSS uses this formula (and I am using
SPSS in my class).
My new stats class is underway and now that I look carefully at the
book (Aron et al) I see that their version of this formula is done
with a denominator of N, which in my opinion causes a underestimation
of dispersion and (for fact) will make my life a living hell if I
choose to go with it.
So I am leaning toward instructing the students to ignore this formula
and use mine. It means that I will not be able to use many of the
practice problems in the book (but I have plenty of others to use) and
might cause them some small amount of confusion. I will probably have
to remind them periodically. Am I being selfish or unfair in trying to
make my life easier this way?
And can someone tell me why most all statistics books have some
feature or formula that is an idiosyncratic version? It almost seems
like things are done whimsically. I've encountered this with
percentiles, stem and leaf and other concepts. This is just the worst
one so far.
Nancy Melucci
Long Beach City College/CSULA
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