|From||Ronán Conroy <firstname.lastname@example.org>|
|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.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).
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.