The desire is to build analytical skills of asking the relevant questions in biostatistics. Are there additional resources to enhance the analytical skills of assessment in biostatistics?

Which study does not need a participant free from the risk factor? For comparison of a treatment or experiment, is it necessary to have a group of participants free of the risk factor? The Cross-Sectional Survey? The Prospective Cohort Study? The Retrospective Cohort Study? The Case-Control Study? The Randomized Controlled Trial? The Crossover Trial? My guess is the Randomized Controlled Trial and The Crossover Trial ? What are your thoughts? Greatly appreciate any comments.