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Re: st: stepwise


From   Richard Williams <Richard.A.Williams.5@ND.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: stepwise
Date   Mon, 04 Sep 2006 09:45:52 -0500

At 04:30 AM 9/4/2006, Timothy.Mak@iop.kcl.ac.uk wrote:

stepwise regression is needed. Say we have n = 200, and a potential pool
of predictors = 50, say that each of these 50 predictors have 1 or 2
missing, not necesarily randomly. Using the Stata stepwise procedure, we
Two other quick comments: Since you are also using SPSS, you could use pairwise deletion of missing data, which might be ok if the data are missing randomly (but if nonrandom, you've got some problems regardless of what you do).

Also, if you've got 50 predictors and are using the .05 level of significance, then just by chance alone you'd expect 2 or 3 vars to enter in. I'd suggest using the .01 level of significance; or figure out exactly what alpha level to use by doing, say, a Bonferroni adjustment.

Overall though, I probably wouldn't feel very comfortable with SW in this case, regardless of how I did it! I'd certainly want to add some cautionary notes in my writeup, perhaps labeling the analysis as exploratory and in need of replication in other studies.

I wonder if some of those 50 vars couldn't be combined into a smaller number of scales? Do you really have 50 unique concepts here, or do you maybe have several items that tap into the same concept in slightly different ways?


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Richard Williams, Notre Dame Dept of Sociology
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