Summary
Walks through case-control study design starting from the why-sample-on-outcome rationale of studying rare diseases efficiently, then defines the study base in primary, secondary, and nested variants. Develops the odds ratio as the cross-product symmetry that survives outcome-based sampling, and unpacks four worked Dohoo examples covering estradiol and breast cancer, salmonella, prostate cancer, and gastroenteritis. Catalogues six control sources with their characteristic biases, distinguishes risk-based from rate-based designs with incidence density sampling, and explains why the rare-disease assumption is needed for some flavors but not others.