Nancy,
I had the same problem with the same book. Ack! Indeed a nightmare. I
ended up teaching both - but unfortunately, you will find that the formula
is imbedded deeply into their presentation (see the warning below). I
changed books for exactly this reason....
WARNING: Back when I used Aron, they presented a z-score method for
calculating a Perason r (rather than the standard covariance formula). The
z-score formula only works when you use N.....if you use N-1 you will get
the wrong answer.
-- Jim
At 01:39 PM 1/8/2007, you 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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