[R] summary of the effects after logistic regression model

From: andrea evangelista <a.evangelist_at_gmail.com>
Date: Thu 25 Jan 2007 - 15:56:01 GMT

Dear all, my aim is to estimate the efficacy over time of a treatment for headache prevention. Data consist of long sequences of repeated binary outcomes (1 if the subject has at least 1 episode of headache , 0 otherwise) on subjects randomized to placebo or treatment.

I have fit a logistic regression model with Huber-White cluster sandwich covariance estimator.
I have put in the model the variables treatment (trt),sex,age and a restricted cubic spline of time (days) to allow for non-linear treatment effects.

I use the functions lrm and robcov from R Design library:

h<-lrm(head ~ trt*rcs(days)+ age+ sex,x=T,y=T) h.rob<-robcov(h,id)

I want to estimate treatment effect over time, then:

k<-contrast(h.rob,list(day=1:240, trt=1),

                          list(day=1:240, trt=0))

xYplot(Cbind(exp(Contrast), exp(Lower),exp( Upper)) ~ day, data=k) #Plot of treatment effects (odds ratio).

The treatment group has a disavantage at the baseline ( for day=1 ,OR=1.16), however at day=210 I can see a reduction of headache risk (OR=0.58) on treatment group.

How can I set to 1 the OR of treatment at the baseline (day=1) with R? In case, is it corrent?

Best regards

Andrea Evangelista

        [[alternative HTML version deleted]]

R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Fri Jan 26 04:14:37 2007

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.1.8, at Fri 26 Jan 2007 - 14:30:30 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help. Please read the posting guide before posting to the list.