STUDY TYPES
This type of study simply describes characteristics of a population without assessing relationships.
What is a descriptive study?
The probability of an event occurring divided by the probability of it not occurring.
What are odds?
This broad class of studies involves no manipulation of exposure.
What are observational studies?
A case-control study reports: 30 cases were exposed, 70 cases unexposed; 10 controls exposed, 90 unexposed. Calculate the odds ratio.
Answer: What is (30/70) ÷ (10/90) = 3.86?
Participants are randomly assigned to receive either a new antihistamine or a placebo, and outcomes are compared prospectively.
Answer: What is a randomized-control study?
This study measures exposure and outcome at one point in time.
What is a cross-sectional study?
This ratio compares the odds of exposure in cases vs controls.
What is an odds ratio?
This type of study lacks full randomization but still involves some form of intervention.
What are quasi-experimental studies?
In a cohort study, the incidence of disease is 15% in the exposed group and 5% in the unexposed group. Calculate the risk difference.
Answer: What is 10%?
Investigators identify 50 patients with lung cancer and 50 without, then look back at their records to see who smoked.
Answer: What is a case-control study?
This observational study selects participants based on outcome and looks back to determine exposure status.
What is a case-control study?
The measure comparing risk in the exposed group to risk in the unexposed group.
What is relative risk (RR)?
This experimental study design randomly assigns participants to treatment groups.
What is a randomized controlled trial (RCT)?
If the risk of outcome in the exposed is 40% and in the unexposed is 20%, calculate the relative risk.
Answer: What is 2.0?
Researchers follow 1,000 exposed miners and 1,000 unexposed office workers for 15 years to measure incidence of COPD.
Answer: What is a cohort study?
In this observational design, participants are selected based on exposure and followed forward to assess outcome.
What is a cohort study?
The absolute difference in risk between exposed and unexposed groups.
What is risk difference / absolute risk reduction?
Matching, stratification, and regression adjustment are used to reduce this common challenge in observational studies.
What is confounding?
A medication reduces stroke risk from 12% to 9%. Calculate the absolute risk reduction (ARR) and number needed to treat (NNT).
Answer: ARR = 3%, NNT = 33.
A hospital introduces a new postoperative protocol and compares complication rates in the six months before and after implementation, but does not use randomization.
Answer: What is a quasi-experimental (before–after) study?
The only design that allows true causal inference through manipulation and randomization.
What is an experimental study / randomized controlled trial?
A cohort study finds: Risk in exposed = 20%, risk in unexposed = 10%. Calculate RR.
What is 2.0?
This type of quasi-experiment uses pre- and post-intervention data but no control group.
What is a before-and-after study?
Given an RCT where: Sensitivity = 90%, Specificity = 80%, Prevalence = 10%. Calculate the positive predictive value (PPV).
PPV = (0.9×0.1) / [(0.9×0.1) + (0.2×0.9)] ≈ 33%.
A researcher surveys 2,000 adults one time to assess whether vaping and chronic cough occur together.
Answer: Cross-sectional study