topic Re: How to get Risk ratio from a Poisson model with count data & interaction terms in Statistical Procedures

How to get Risk ratio from a Poisson model with count data & interaction terms

JME1 — Wed, 23 Sep 2020 14:40:20 GMT

Hello!

I'm having a bit of trouble getting RR from a Poisson model using count data and with interaction terms. I have data like this:

data x;

count time intervention status totalcount;

12 1 1 0 21

34 2 0 0 40

12 3 0 1 20

15 4 1 1 20

16 5 0 1 21

run;

*Note: Count is number of patients hospitalized, and total count is total patients.*

I'm interested in knowing how the intervention affected the count, and also how it differed by status so I ran this model.

proc genmod data=x;
class status intervention/param=glm;
model count= time intervention status status*intervention intervention*time/dist=poisson offset=log_tc;
run;

*log_tc*= I took the log of the total count*

I get the coefficients from this output but I want to be able to interpret my output in terms of risk ratios/rate ratios.

However, using the estimate statements/LSMeans does not exactly work in this situation and I'm not sure what to do to get it since I have interaction terms in my model.

If I had just main effect terms in the model, I would take the exponents of the coefficients, but with interaction terms how would I get the estimates? and interpret all of them, including the interactions?

For instance I want to know how the count varied by status (=1) where intervention=1.

I would appreciate any guidance on this!

Re: How to get Risk ratio from a Poisson model with count data & interaction terms

ChrisNZ — Thu, 01 Oct 2020 22:55:19 GMT

Moved to STATS forum.

Re: How to get Risk ratio from a Poisson model with count data & interaction terms

StatDave — Fri, 02 Oct 2020 15:07:19 GMT

Your response is apparently binary and then aggregated, resulting in a count of events out of a total count of trials. As such you can simply fit a logistic model using "events/trials" syntax for specifying the observed counts. You can then obtain the risk ratio (relative risk) using the NLMEANS macro as discussed and illustrated in this note.

Re: How to get Risk ratio from a Poisson model with count data & interaction terms

JME1 — Tue, 13 Oct 2020 19:59:38 GMT

Thank you!