[R] General help on sample size based on recurrent events

[si.fu at ntlworld.com]

Hello,
 
I have a general help question that some of you might be able able to help.

I would like calculate sample size for a study (a two group comparison) based on the outcome of reduction on (recurrent) events (say hospital admissions). 

In a previous study, hospital admission rate of 140 admissions per 72 patients (over a 4 month period) has been observed. That is rate is about 1.9. In order to see the admission rate reduction of 50% over a 4 month period, i.e. 0.95, in a two group comparison (alpha 0.05) with appropriate power (say 80%) what sort of group sizes are needed.
 
How would I go about this?

Many Thanks,
 
Si.