Power calculation for a case-control study with a continuous outcome

Question

We're planning a case-control study, in which we assign subjects to be either cases or controls (based on normal vs low birth weight) and then measure a continuous outcome variable (% body fat) subsequently.

Can anyone point me towards a method for calculating how many cases vs controls we will need. We have an idea of the standard deviation of the outcome variable (% body fat).

Many thanks

Andrew

Answer 1

As I said in my comment, too much information (e.g., expected % of dropouts or missing values, type of matching, one-sided or two-sided hypothesis) is missing to propose a 'magic' formula. So, for general references (apart from classical epidemiological textbook), I would recommend:

• Donner, A. Approaches to sample size estimation in the design of clinical trials–A review, Statistics in Medicine (1984) 3:199.
• Edwardes, MD. Sample size requirements for case-control study designs, BMC Medical Research Methodology (2001) 1:11.

Answer 2

Reading about power analysis in any experiment design textbook might answer your question. But here's the general idea: You have a few unknowns relating to power

• Variance
• Sample size
• p-value

So you try to guess what the unknowns are.

1. Run a pilot study so that you can guess what the variance would be.
2. Choose alpha.
3. I feel like I missed one of the unknowns. If I did, you figure that out to.
4. Do the statistical computations in reverse to solve for sample-size. You can also generate random data based on the variance and different sample sizes and then run the statistical computations the normal way, but the analytical approach will probably be easier for the simple design that you have.

And you can of course graph this so you get an idea of how robust your decision is to variations in the variance.