An epidemiological research team conducts a retrospective cohort study to investigate the … — USMLE Step 2 CK

An epidemiological research team conducts a retrospective cohort study to investigate the association between residential proximity to major highways and the development of chronic obstructive pulmonary disease (COPD) in adults. The initial unadjusted analysis yields a Risk Ratio (RR) of $1.7$ ( $95\%$ CI: $1.3 - 2.2$ ) for developing COPD among individuals living within $100$ meters of a major highway compared to those living further away.

To account for potential alternative explanations, the researchers perform a stratified analysis based on household income level:

Income Level	Risk Ratio ( $95\%$ CI)
Low-income	$1.1$ ( $0.8 - 1.4$ )
High-income	$1.0$ ( $0.7 - 1.3$ )

Based on these findings, which of the following epidemiological concepts best describes the role of household income in the original association?

A
Berkson paradox
B
Effect modification
ConfoundingAnswer
D
Lead-time bias
E
Attrition bias

Answer

Confounding

The correct answer is confounding. Confounding occurs when a third variable is associated with both the exposure and the outcome, skewing the apparent relationship between them. In this scenario, lower household income may be associated with both living near a highway and higher rates of COPD (e.g., due to higher occupational exposures or smoking rates). When researchers stratify the data by income, they control for its effect. Because the stratum-specific Risk Ratios (

1.1

and

1.0

) are similar to each other and both approach the null value, it indicates that the crude Risk Ratio of

1.7

was falsely elevated by the confounding effect of household income, rather than a true direct association with highway proximity.

Step-by-Step Solution

Identify the unadjusted (crude) association from the study data.

The crude Risk Ratio is

1.7

, indicating a statistically significant positive association between highway proximity and COPD.

Establishing the baseline finding is necessary before evaluating the impact of stratification.

Analyze the results of the stratified analysis based on household income.

In both the low-income and high-income groups, the Risk Ratios fall to

1.1

and

1.0

respectively, which are non-significant and close to the null value of

1.0

Stratification reveals how the measure of association behaves within isolated subgroups of a potential third variable.

Compare the crude Risk Ratio to the stratum-specific Risk Ratios to determine the epidemiological phenomenon.

The stratum-specific Risk Ratios are similar to each other (

1.1

and

1.0

) but distinctly different from the crude Risk Ratio (

1.7

This specific pattern—where the association largely disappears upon stratification—indicates that the original crude association was driven by the stratifying variable.

Conclude the role of the stratifying variable.

Household income acts as a confounder in the relationship between residential highway proximity and COPD.

A confounder is associated with both the exposure and the outcome, and controlling for it via stratification removes the distorted crude association.

Key Concept

Confounding is identified when the stratum-specific measures of association are similar to one another but notably different from the crude (unadjusted) measure of association.

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