Nancy, Later on this text Aron et al do introduce the N-1 version of the formula for SD. (I am at home and the book is on my desk in the office so I can't look up the chapter.) At that point they explain why dividing by N-1 gives an unbiased estimate of the population variance (and standard deviation) from a sample. In earlier editions of this textbook the population formula (dividing by N) was very important because they used the average of the cross products of Z scores to calculate r (r=Sum(Zx*Zy)/N) and that formula only works when the Z scores are calculated with SD's that are calculated by dividing by N. The current edition uses a different formula and introduces correlations later in the text.
Good luck Dennis -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Mon 1/8/2007 2:39 PM To: Teaching in the Psychological Sciences (TIPS) Subject: [tips] A statistical crisis or nightmare 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 --- To make changes to your subscription go to: http://acsun.frostburg.edu/cgi-bin/lyris.pl?enter=tips&text_mode=0&lang=english
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