When to contact us
Contact at the earliest possible time, usually more than 6 weeks prior to any deadline. This time frame is needed because it is often time consuming for you to find the needed information for the calculations.
What to expect
After a scope of work is developed and signed, be prepared to spend much of a first meeting with the collaborating biostatistician discussing your science and study design before moving onto a discussion on sample size. The end product is a table of sample size choices and a written paragraph justifying the sample size.
Key data needed from you
Sample size is a function of:
- variation in outcome measures (and sometimes primary predictors),
- the size of a clinically meaningful difference between groups,
- level of significance (are multiple tests being performed),
- and the desired type II error (the probability that you will not find a difference when a difference, in truth, exists).
You will need to provide from historical literature, pilot data, or other clinical information the following information.
- Measures of variation (standard deviations) of the outcome measures.
- An estimate of a clinically meaningful difference between groups or an estimate of the size of association that is clinically meaningful (e.g., correlation).
- In studies with repeatedly collected measurements on an individual (or cluster) estimates of correlation within individuals (or clusters) is also needed.
- For studies with a continuous variable of interest (e.g., age) the variation of the variable of interest is also needed.
The lead biostatistician cannot perform a valid literature search for you. Finding this information is something that only those with scientific expertise in the field can adequately do.