Right - I know that. But, that has always been the case and we have only
recently been using the "N" formula at all. Look back ten years and you
will find that most texts never even introduce the "N" formula. McCall's
2001 edition (just released) still only uses the "N-1" formula.
Apparently, other disciplines use N-1 as well. This is the first semester
in which I have broken down and followed the text, using only the "N"
formula at the start - and now my students are telling me they use N-1 in
Chemistry and Biology!
So - my question remains: Why the change? As I noted, I suspect it has
to do with using the z-score formula for r. In response to an earlier
note, it may be true that the z-score formula is more intuitive for the
professor - but it is certainly not more intuitive for the students, most
of whom have absolutely no idea what a "moment" might be. Raw score
deviation formulas make sense for the students because they understand a
deviation score. The z-score formula, just one step removed from the
deviation formula, is sufficiently abstract to confuse the beginning
student.
-- Jim
On Mon, 25 Sep 2000, Larry Z. Daily wrote:
> The N version of the formula is a descriptive statistic - it simply
> describes the current sample (or population). The N-1 version is an
> inferential statistic and takes into account degrees of freedom and the fact
> that I've estimated the mean of the population.
>
> Larry
>
> ************************************************************
> Larry Z. Daily
> Assistant Professor of Psychology
> Department of Psychology
> White Hall, Room 213
> Shepherd College
> Shepherdstown, West Virginia 25443
>
> phone: (304) 876-5297
> email: [EMAIL PROTECTED]
> WWW: http://webpages.shepherd.edu/LDAILY/index.html
>
>
> > -----Original Message-----
> > From: James D. Dougan [mailto:[EMAIL PROTECTED]]
> > Sent: Saturday, September 23, 2000 4:00 PM
> > To: [EMAIL PROTECTED]
> > Subject: Another Standard Deviation question
> >
> >
> > All of this standard deviation talk suggests it is a good time to ask a
> > question which has been bothering me for the last couple of years.....
> >
> > Back in the "good old days" all (or at least most) of the undergraduate
> > statistics texts taught the standard deviation using ther "N-1" formula.
> > The "N" formula was perhaps mentioned in a footnote, but often not
> > mentioned at all...
> >
> > Now, virtually all of the texts teach the "N" formula in the beginning
> > under descriptive stats, then introduce N-1 later under inferential.
> >
> > I hate the new way of doing it, partly because I have to remember new
> > formulas but more because it seems to confuse the students.
> >
> > Why the change? What was wrong with the old method?
> >
> > My guess is that using the "N" formula allows one to use the z-score
> > formula for Pearson r (the z-score formula for r does not work if you use
> > N-1, a fact I unfortunately discovered in the middle of a lecture
> > demonstration...).
> >
> > But - why use the z-score formula for r? The old covariance
> > formula is far
> > more intuitive than the z-score formula. Yes - you can easily
> > show how the
> > product of z-scores works the same way as covariance, but students really
> > don't grasp z-scores very well to begin with and it is hard for
> > them to get
> > the translation back from z to covariance.
> >
> > My guess is that texts want to use the z-score formulation for r because
> > it makes a smoother transation into multiple regression. But (sigh) I
> > don't think multiple regression really belongs in a 200-level intro stat
> > course....
> >
> > Lots of room for answers and commentaries here....shoot away....
> >
> > -- Jim
> >
> >
> >
> >
> >
>
>
