Study design V: Case-Control Studies Principles of design and threats to validity Brent Hagel, PhD

Case-control study Outcome based selection Cases have desired characteristic: Disease OR other outcome Controls don’t Analysis compares groups for: Frequency of risk factors Retrospective OR prospective

Case-control Studies Subjects are selected on outcome status (disease / no disease) Look back to determine exposure status There can be prospective data collection in a case-control study Exposure Disease ? Time ? Study Begins From Hennekens & Buring. Epidemiology in Medicine. Little Brown and Company, 1987; P. 24

What is the relation between maternal smoking and birth defects? Past Present Maternal smoking Yes? No? Case: Infant with a Birth Defect primary analysis Maternal smoking Yes? No? Control: “Normal Infant”

Retrospective case-control study - strengths Useful for rare outcomes Less expensive and labour intensive than cohort study with same outcome Efficient for diseases with long induction/latency period before disease development Useful for hypothesis generation

Retrospective case-control study - weaknesses Cannot estimate prevalence or incidence Can be difficult to establish temporal relationship Only a single outcome can be studied Susceptible to information bias Reporting, historical sources of information, recall

Prospective case-control study Strengths over retrospective design Prospective identification of subjects and data collection (exposure) Standardisation of outcome Weaknesses No estimate of prevalence/incidence Single outcomes Waiting for sufficient outcomes to accrue (time and resources)

Threats to validity Selection bias? Misclassification bias? e.g., Control group = infants with respiratory problems? Misclassification bias? e.g., Differential recall for cases and controls? Confounding? e.g., mother’s alcohol use?

Design a case-control study . . . Research Question: “Are bicycle helmets effective in preventing head injuries?” You have 30 minutes!

Cheat sheet Choice of design reasons for using a case-control study reasons against a case-control study Who would be an ideal case? Who would be an ideal control? Where would you obtain cases and controls? What would be your main exposure variable and how would you measure it? What other variables would you consider in this study?