They do have the sample formula later, when they are doing hypothesis testing.
At 01:59 PM 1/8/2007, you wrote:
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
<mailto:[EMAIL PROTECTED]>[EMAIL PROTECTED]
http://www.yorku.ca/christo
"All warfare is based on deception."
Sun-tzu, The Art of War, I.18
=============================
<mailto:[EMAIL PROTECTED]>[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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