Observational studies collect data without assigning treatments or exposures, so they are essential when experiments are unethical, impossible, or too expensive. In statistics and health science, three major observational designs are cohort, case-control, and cross-sectional studies. Each design answers a different kind of question about exposure, outcome, time, and risk.
Choosing the right design helps researchers make stronger conclusions from real-world data.
Key Facts
- Cohort studies group people by exposure status and follow them to measure later outcomes.
- Case-control studies start with outcome status and look backward to compare past exposures.
- Cross-sectional studies measure exposure and outcome at one point in time to estimate prevalence.
- Risk = number with outcome / total number in group.
- Relative risk = risk in exposed group / risk in unexposed group.
- Odds ratio = odds of exposure among cases / odds of exposure among controls.
Vocabulary
- Observational study
- A study in which researchers observe variables without assigning treatments or exposures.
- Cohort study
- A study that follows exposed and unexposed groups over time to compare how often an outcome occurs.
- Case-control study
- A study that compares people with an outcome to people without it and looks back for differences in exposure.
- Cross-sectional study
- A study that measures exposure and outcome at a single time point in a population.
- Confounding variable
- A variable related to both the exposure and the outcome that can distort the apparent relationship between them.
Common Mistakes to Avoid
- Calling every observational study a survey is wrong because cohort, case-control, and cross-sectional designs differ in timing, grouping, and what they estimate.
- Using relative risk for a case-control study is wrong in most cases because the number of cases and controls is chosen by the researcher, so risks cannot usually be calculated directly.
- Treating association as causation is wrong because observational studies can be affected by confounding variables, bias, and reverse causality.
- Ignoring the time direction of the design is wrong because cohort studies usually move from exposure to outcome, case-control studies move from outcome to past exposure, and cross-sectional studies measure both at once.
Practice Questions
- 1 In a cohort study, 80 out of 400 exposed people develop a disease, while 40 out of 500 unexposed people develop it. Calculate the risk in each group and the relative risk.
- 2 In a case-control study, 90 of 150 cases had a past exposure, while 60 of 200 controls had the exposure. Calculate the odds of exposure in each group and the odds ratio.
- 3 A researcher measures screen time and current sleep quality for 1,000 students during one week and finds that students with higher screen time report worse sleep. Identify the study design and explain why this result alone does not prove that screen time caused worse sleep.