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Re: st: RRR with CI from logit model

From   Ron´┐Żn Conroy <[email protected]>
To   [email protected]
Subject   Re: st: RRR with CI from logit model
Date   Tue, 02 Nov 2004 10:35:35 +0000

Michael Ingre wrote:

A follow up on statistical power.

I have calculated a few RRRs and an interesting pattern is emerging. Extreme comparisons give insignificant p-values but others don't.

RRR for 9.5 vs 1, p=.669
RRR for 9.5 vs 9, p=.030
RRR for 2 vs 1, p=.049

Predicted absolute probabilities are: 9.5=.33 , 9=.14, 2=.000020 & 1=.000015

What is going on here? Am I doing something wrong? I appreciate any suggestion because this makes no sense to me.
Take a step back here. Have you *graphed* your outcome against your predictor variable? Use a smoother to have a look at the shape of the relationship. I sometimes use -autosmoo-, but usually do this sort of thing in JMP, where you can vary the smoothness of a spline interactively. It is handy to know if there is a threshold effect (above a critical value, risk begins to rise) or even a 'normal region' phenomenon, whereby risk is lowest in some normal region, and rises at the high and low extremes (weight and health is a classic example).

You may also be the victim of small numbers in some of the categories.

But relative risk ratios are a way of measuring a phenomenon. The first thing to do is to inspect the phenomenon personally, using the Mk I intra-ocular traumatic test.

Ronan M Conroy ([email protected]) Senior Lecturer in Biostatistics Royal College of Surgeons Dublin 2, Ireland +353 1 402 2431 (fax 2764) -------------------- Just say no to drug reps

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