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RE: st: stata and spss output
On Mon, 31 Mar 2003, Nick Winter wrote:
> > -----Original Message-----
> > From: Ernest Berkhout [mailto:firstname.lastname@example.org]
> > Sent: Monday, March 31, 2003 10:05 AM
> > To: email@example.com
> > Subject: Re: st: stata and spss output
> > At 10:24 31-3-2003, you wrote:
> > > From
> > >memory, SPSS was doing things incorrectly; it was using the weighted
> > >sample size, rather than actual number of obs. (Stata got things
> > >right.)
> > There have been reports of SPSS sometimes 'ignoring' very low
> > weighted
> > frequencies... (I actually saw it doing that)
> At least under some conditions, SPSS seems to round weights to the
> nearest integer.
> My experience leads me to say that SPSS should be avoided at all costs,
> especially when you've got survey weights.
Indeed. I was forced to use SPSS to teach fellows in an intro epi program
once. After presenting the 2x2 table, odds ratio, and why the OR is used
so much in epidemiology, I moved on to the logistic model to discuss how
adjusted ORs are obtained. When I came to the ta-DA moment demonstrating
that the OR from the logistic model is identical to that from a 2x2 when
only the single dichotomous predictor is entered in the model, I came up
with a different OR in SPSS. After much exploring and consulting with
more mathematically sophisticated friends, we found that the SPSS
algorithms simply stop short of the last iteration in some circumstances.
Since ad/bc IS the MLE, they should be identical. As this is one of the
few situations where the precise calculation is easy, I've become wary of
SPSS when there is no easy check.
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