Sample Size for Retrospective Comparative Study

[Emily Scott]

A previous database comparing two procedures, n1 = 47 and n2 = 206.  We are updating the databases with more patients, and we need to know how many patients we need for n1 given a new n2 of 350.  What equation will be best for determining n1?

 

Thank you!

[Lenth, Russell V]

It depends on your goals, e.g. how accurately you want to estimate whatever it is you want to estimate – and I have no idea because such goals are not stated. I guess if your goal is simply to have the same proportional representation of the groups, you need n1 = 47 * 350 / 206. But that’s just algebra, not statistics.

 

Russell V. Lenth  -  Professor Emeritus

Department of Statistics and Actuarial Science  

The University of Iowa  -  Iowa City, IA 52242  USA  

Voice (319)335-0712 (Dept. office)  -  FAX (319)335-3017

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