You are conducting a study comparing the efficacy of two different statin medications. Two groups are placed on different statin medications, statin A and statin B. Baseline LDL levels are drawn for each group and are subsequently measured every 3 months for 1 year. Average baseline LDL levels for each group were identical. The group receiving statin A exhibited an 11 mg/dL greater reduction in LDL in comparison to the statin B group. Your statistical analysis reports a p-value of 0.052. Which of the following best describes the meaning of this p-value?

There is a 5.2% chance of observing a difference in reduction of LDL of 11 mg/dL or greater even if the two medications have identical effects

There is a 95% chance that the difference in reduction of LDL observed reflects a real difference between the two groups

Though A is more effective than B, there is a 5% chance the difference in reduction of LDL between the two groups is due to chance

If 100 permutations of this experiment were conducted, 5 of them would show similar results to those described above

This is a statistically significant result

A 6-month-old male presents for a routine visit to his pediatrician. Two months ago, the patient was seen for tachypnea and wheezing, and diagnosed with severe respiratory syncytial virus (RSV) bronchiolitis. After admission to the hospital and supportive care, the patient recovered and currently is not experiencing any trouble breathing. Regarding the possibility of future reactive airway disease, which of the following statements is most accurate?

“Your child has a greater than 20% chance of developing asthma”

“There is no clear relationship between RSV and the development of asthma.”

“Your child’s risk of asthma is less than the general population.”

“Your child has a less than 5% chance of developing asthma”

“Your child’s risk of asthma is the same as the general population.”

A research fellow proposes a nested case-control study within an existing cohort examining antibiotic exposure and C. difficile infection. The mentor suggests this design wastes the cohort structure and that relative risk should be calculated instead. The fellow argues that odds ratios from nested case-control studies approximate relative risk while being more efficient. Evaluate the validity of each position and synthesize the optimal approach.

Use nested case-control with OR since it's mathematically equivalent to cohort RR with proper sampling

Calculate RR from full cohort since all data are available and RR is more interpretable

Use nested case-control only if computational resources are limited, otherwise use full cohort

Perform both analyses and compare results to validate the nested case-control approach

Calculate hazard ratios using Cox regression as a compromise between efficiency and accuracy

Two studies examine statin therapy and stroke prevention. Study A (cohort, n=10,000) reports RR=0.75. Study B (case-control, n=2,000) reports OR=0.68. The absolute stroke rate in the general population is 2% over 5 years. Analyze these findings to determine which study provides more accurate information for clinical decision-making and why.

Study A because the cohort design allows calculation of absolute risk reduction

Study B because case-control designs eliminate confounding

Study B because odds ratios are more generalizable across populations

Study A because relative risk is always more accurate than odds ratio

Both are equally valid since stroke is a rare outcome making OR approximate RR

A genetic epidemiology study uses a case-control design to examine BRCA1 mutations and breast cancer risk, reporting an odds ratio of 15.0 (95% CI: 8.2-27.4). A patient with a BRCA1 mutation asks what this means for her actual risk of developing breast cancer. Evaluate how to appropriately counsel this patient regarding the study findings.

The odds ratio cannot be directly interpreted as risk; population incidence data are needed

The confidence interval suggests her risk is between 8-27 times higher

Convert OR to RR using the formula: RR = OR/(1 + P0(OR-1)) where P0 is baseline risk

Her risk is 15 times higher than women without the mutation

She has a 15% absolute risk of developing breast cancer

Interpretation differences — USMLE Lesson

Relative Risk (RR) - The Cohort Compass

Definition: Compares the risk of developing a disease in the exposed group to the risk in the unexposed group. The primary measure for cohort studies.
Calculation: Based on incidence.
- Formula: $RR = \frac{\text{Incidence in exposed}}{\text{Incidence in unexposed}} = \frac{[a/(a+b)]}{[c/(c+d)]}$
Interpretation:
- $RR > 1$: ↑ risk in the exposed group. (e.g., RR = 2.5 means a 150% increase in risk).
- $RR < 1$: ↓ risk in the exposed group (protective exposure).
- $RR = 1$: No association between exposure and outcome.

📌 Mnemonic: Relative Risk is for cohoRRt studies.

2x2 Table for Relative Risk Calculation (Cohort Study)

⭐ RR provides a direct measure of risk. A statement like "smokers are 20 times more likely to develop lung cancer" is a statement of relative risk.

Odds Ratio (OR) - The Case-Control Clue

Primary Use: The go-to measure for case-control studies. We start with the outcome (cases vs. controls) and retrospectively assess for prior exposure.
Core Question: "What are the odds that a case was exposed versus the odds that a control was exposed?"
Calculation: From a 2x2 table, the OR is the ratio of the odds of exposure in cases to the odds of exposure in controls.
- Formula: $OR = (a/c) / (b/d) = ad/bc$
Interpretation:
- OR > 1: Increased odds of exposure among cases.
- OR < 1: Decreased odds of exposure among cases (protective exposure).

⭐ When disease prevalence is low (<10%), the OR closely approximates the Relative Risk (RR). As prevalence increases, the OR tends to overestimate the RR.

2x2 table for odds ratio calculation

Interpretation - When Odds Aren't Risky

Odds Ratio (OR) and Relative Risk (RR) measure the association between an exposure and an outcome. Their interpretations differ, especially with common diseases.

Value	Odds Ratio (OR) Interpretation	Relative Risk (RR) Interpretation
> 1	Increased odds of outcome with exposure	Increased risk (probability) of outcome
< 1	Decreased odds of outcome with exposure	Decreased risk (probability) of outcome
= 1	No association between exposure & outcome	No association between exposure & outcome

-   OR is a good approximation of RR when the disease is rare (prevalence < **10%**).
-   As disease prevalence ↑, the OR increasingly *overestimates* the RR.
-   📌 **OR** **O**verestimates **R**isk.

⭐ Exam Favorite: Case-control studies, which sample based on disease status, can only calculate an Odds Ratio. Prospective cohort studies can calculate both Relative Risk and Odds Ratio.

Odds Ratio (OR) is the primary measure for case-control studies; it compares the odds of exposure in cases vs. controls.

Relative Risk (RR) is the standard for cohort studies; it compares the risk of disease in exposed vs. unexposed groups.

When disease prevalence is low (<10%), the OR approximates the RR.

With higher prevalence, the OR will overestimate the RR.

RR is more intuitive: "twice the risk."

OR is less direct: "twice the odds."

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