Question 1. Is the study descriptive or is the study comparative? Descriptive studies include surveys to assess prevalence, needs assessments, chart reviews, etc. that have as a main aim the estimation of rates, proportions and means in a population with a secondary aim being to examine whether the rates are related to demographic variables (i.e. a correlational analysis). For example, a survey may be undertaken to assess the extent of doughnut consumption in Belltown. The main results to be reported might be the percentage of residents who consume doughnuts on a daily basis, or the mean number of doughnuts consumed by a resident per week. Follow-up analysis might examine whether the consumption rates depend on the sex or age of the resident. Sample size determination for descriptive studies is based on confidence intervals; that is, the level of precision required in providing estimates of the rates, proportions and means.
Comparative studies include case-control designs, randomized clinical trials, etc. where a comparison between two or more groups is the key analysis. The main aim here is to establish whether there are statistically significant differences between groups with respect to some key outcome variable. Sample size determination for comparative studies is based on hypothesis tests and power, that is, the probability of being able to find differences when they do, in fact, exist.
In brief, the first question asks, “Are P-values relevant here?”
Note that descriptive studies often lead to comparative studies; in fact, post hoc analysis often involves informal inference and model-building to examine relationships among variables. The sample size estimation should also take this into account and be sufficiently large for analysis of future questions.