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Difficulty: HardMeasures of Disease Frequency and Association

A 10-year prospective cohort study is conducted to evaluate the association between chronic shift work (defined as working primarily night shifts) and the incidence of type 2 diabetes mellitus. The study enrolls a total of 20,00020,000 adult participants who do not have diabetes at baseline. Among these participants, 5,0005,000 are classified as chronic shift workers, and 15,00015,000 are classified as regular day-shift workers. Over the 10-year follow-up period, 600600 of the chronic shift workers and 600600 of the day-shift workers develop type 2 diabetes mellitus. Assuming that this association is causal, which of the following is the percentage of type 2 diabetes cases among the chronic shift workers that can be attributed to their shift work?

  1. A
    8%8\%
  2. B
    33%33\%
  3. 67%67\%Answer
  4. D
    200%200\%
  5. E
    300%300\%

Answer

The correct answer is 67%67\%.
The question asks for the Attributable Risk Percent (ARP), which is the percentage of disease in the exposed group that can be attributed to the exposure. First, calculate the incidence in the exposed (Iexp=600/5,000=12%I_{exp} = 600 / 5,000 = 12\%) and the incidence in the unexposed (Iunexp=600/15,000=4%I_{unexp} = 600 / 15,000 = 4\%). The formula for ARP is (Iexp−Iunexp)/Iexp(I_{exp} - I_{unexp}) / I_{exp}. Substituting the values yields (12%−4%)/12%=8%/12%=2/3(12\% - 4\%) / 12\% = 8\% / 12\% = 2/3, which is approximately 67%67\%. This means that 67%67\% of the type 2 diabetes cases occurring in the chronic shift workers are attributable to their shift work.

Step-by-Step Solution

1
Calculate the incidence of type 2 diabetes in the exposed group (chronic shift workers).
Iexp=6005000=0.12I_{exp} = \frac{600}{5000} = 0.12 (12%12\%).
Determining the absolute risk in the exposed cohort is the first requirement for calculating any measure of association.
2
Calculate the incidence of type 2 diabetes in the unexposed group (day-shift workers).
Iunexp=60015000=0.04I_{unexp} = \frac{600}{15000} = 0.04 (4%4\%).
The baseline risk in the unexposed group must be known to identify the excess risk caused by the exposure.
3
Identify the requested epidemiological metric based on the question text.
The question asks for the "percentage of cases among the chronic shift workers that can be attributed to their shift work," which corresponds to the Attributable Risk Percent (ARP).
Precise identification of the metric is crucial, as the data can yield multiple different valid epidemiological measures (e.g., AR, PARP, RR).
4
Calculate the Attributable Risk Percent (ARP).
ARP=Iexp−IunexpIexp=0.12−0.040.12=0.080.12=23≈66.7%ARP = \frac{I_{exp} - I_{unexp}}{I_{exp}} = \frac{0.12 - 0.04}{0.12} = \frac{0.08}{0.12} = \frac{2}{3} \approx 66.7\%.
This formula divides the excess risk (Attributable Risk) by the total risk in the exposed group to find the fraction of disease directly attributable to the exposure.

Key Concept

Attributable Risk Percent (ARP) represents the fraction of disease in the exposed group that is directly attributable to the exposure, calculated as (Iexp−Iunexp)/Iexp(I_{exp} - I_{unexp}) / I_{exp}.
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Topics

Measures of Disease Frequency and Association

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