You have been asked to quantify the relative risk of developing bacterial meningitis following exposure to a patient with active disease. You analyze 200 patients in total, half of which are controls. In the trial arm, 30% of exposed patients ultimately contracted bacterial meningitis. In the unexposed group, only 1% contracted the disease. Which of the following is the relative risk due to disease exposure?

[30 / (30 + 70)] / [1 / (1 + 99)]

[70 / (30 + 70)] / [99 / (1 + 99)]

[[1 / (1 + 99)] / [30 / (30 + 70)]]

You are reviewing the protocol for a retrospective case-control study investigating risk factors for mesothelioma among retired factory workers. 100 cases of mesothelioma and 100 age and sex matched controls are to be recruited and interviewed about their exposure to industrial grade fiberglass by blinded interviewers. The investigators' primary hypothesis is that cases of mesothelioma will be more likely to have been exposed to industrial grade fiberglass. The design of this study is most concerning for which type of bias?

This study design is free of potential bias

Which of the following study designs would be most appropriate to investigate the association between electronic cigarette use and the subsequent development of lung cancer?

Subjects who smoke electronic cigarettes and subjects who do not smoke

Subjects with lung cancer who smoke and subjects with lung cancer who did not smoke

Subjects who smoke electronic cigarettes and subjects who smoke normal cigarettes

Subjects with lung cancer who smoke and subjects without lung cancer who smoke

Subjects with lung cancer and subjects without lung cancer

Study X examined the relationship between coffee consumption and lung cancer. The authors of Study X retrospectively reviewed patients' reported coffee consumption and found that drinking greater than 6 cups of coffee per day was associated with an increased risk of developing lung cancer. However, Study X was criticized by the authors of Study Y. Study Y showed that increased coffee consumption was associated with smoking. What type of bias affected Study X, and what study design is geared to reduce the chance of that bias?

Observer bias; double blind analysis

A 16-year-old girl comes to the physician because she is worried about gaining weight. She reports that at least twice a week, she eats excessive amounts of food but feels ashamed about losing control soon after. She is very active in her high school's tennis team and goes running daily to lose weight. She has a history of cutting her forearms with the metal tab from a soda can. Her last menstrual period was 3 weeks ago. She is 165 cm (5 ft 5 in) tall and weighs 57 kg (125 lb); BMI is 21 kg/m2. Physical examination shows enlarged, firm parotid glands bilaterally. There are erosions of the enamel on the lingual surfaces of the teeth. Which of the following is the most likely diagnosis?

Borderline personality disorder

Case-control studies for USMLE Step 2 CK: High-Yield Behavioral Science Lessons

Case-Control 101 - Hindsight Detectives

A retrospective study design that starts with an outcome (disease) and looks backward in time to find the exposure.

Compares the odds of prior exposure between the two groups.
Key metric: Odds Ratio (OR), calculated as $OR = (ad)/(bc)$.

⭐ Highly susceptible to recall bias, as cases (diseased) may remember past exposures differently than controls (healthy).

Case-control study design showing retrospective analysis

📌 Case-Control starts with the Consequence (disease) to find the Cause.

Odds Ratio - The Betting Statistic

Represents the odds that a case was exposed compared to the odds that a control was exposed.
Calculated from a 2x2 table: $OR = (a/c) / (b/d) = ad/bc$.
- a: cases exposed
- b: controls exposed
- c: cases unexposed
- d: controls unexposed
Interpretation:
- OR > 1: ↑ Odds of exposure in cases (risk factor).
- OR = 1: No association.
- OR < 1: ↓ Odds of exposure in cases (protective factor).

⭐ In rare diseases (low prevalence), the Odds Ratio approximates the Relative Risk (RR).

2x2 table, odds ratio, and 95% CI for case-control study

Pros & Cons - The Give and Take

Pros (Advantages)	Cons (Disadvantages)
* Quick & Inexpensive: Less time and resources needed compared to cohort studies.	* Recall Bias: Cases may remember exposures differently than controls.
* Rare Diseases: Excellent for studying diseases with low prevalence.	* Selection Bias: Controls may not be representative of the exposure in the population.
* Multiple Exposures: Can investigate many potential risk factors at once.	* Temporality Issues: Cannot definitively prove exposure preceded the disease.
* Fewer Subjects: Smaller sample sizes are typically required.	* Cannot Calculate Incidence/Prevalence: Only provides an odds ratio.

Common Biases - The Usual Suspects

Selection Bias: Controls are not representative of the population that produced the cases.
- Berkson's bias: Hospital-based controls differ from the general population.
- Neyman bias (prevalence-incidence): Excludes fatal or short-lived cases.
Recall Bias: Cases may remember past exposures more accurately or vividly than controls.

⭐ Matching cases to controls on potential confounders (e.g., age, sex) is a key strategy to increase study validity.

High‑Yield Points - ⚡ Biggest Takeaways

A retrospective design, looking back in time from disease to exposure.

Excellent for studying rare diseases or those with long latency periods.

Compares a group with the disease (cases) to a similar group without it (controls).

The primary measure of association is the odds ratio (OR).

Cannot be used to calculate prevalence or incidence.

Major weaknesses include recall bias and selection bias.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play