Re: st: RRR with CI from logit model

[Ron�n Conroy <rconroy@rcsi.ie>]

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 (rconroy@rcsi.ie) 
Senior Lecturer in Biostatistics 
Royal College of Surgeons 
Dublin 2, Ireland 
+353 1 402 2431 (fax 2764) 
-------------------- 
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