Hello,

I am assisting in the early planning stages of a three year lab study and I am looking for the best formula to calculate the required sample size.

The study will record a measure of respirator fit at six month intervals. Up to nine replications will be recorded at each time period (and then averaged) because of the high variance of this test. At each time period, we will be collecting weight/facial measurements and a variety of demographic and health data to see if any of theses factors are related to respirator fit.

mrc

If you type the following search terms in Google, you'll get a lot of links to resources that talk about sample size considerations in longitudinal studies:

"sample size"
"longitudinal study"

Unfortunately, there are a lot of considerations, including:

- anticipated drop-out rate
- subject-to-subject variability
- within-subject variability
- etc.