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Difficulty: HardMeasures of Association and Impact

An occupational medicine specialist is reviewing health data from a large manufacturing corporation to evaluate the impact of chronic noise exposure on the development of sensorineural hearing loss.

The database includes a 5-year longitudinal cohort of 20,000 factory employees who had normal hearing at baseline. Occupational health records classify employees into two groups: those working in "high-noise" zones (exposed) and those working in administrative or "low-noise" zones (unexposed).

At the end of the 5-year period, audiometric testing yielded the following new-onset sensorineural hearing loss cases:

Exposure StatusTotal Employees in GroupNew Hearing Loss Cases
High-Noise5,000400
Low-Noise15,000600

The corporate medical director wants to determine the proportion of all incident cases of sensorineural hearing loss in the entire workforce that could theoretically be prevented if high-noise exposure were completely eliminated.

What is the population attributable risk percent (PAR%) of sensorineural hearing loss due to high-noise exposure in this workforce? (Express your answer as a whole number).

Answer: 20 %

Answer

20
The correct answer is 20. The Population Attributable Risk Percent (PAR%) indicates the proportion of incident cases in the entire population that are attributable to the exposure. First, calculate the overall incidence (ItotalI_{total}) which is 1,000 total cases divided by 20,000 total employees (0.05). Next, calculate the incidence in the unexposed group (IunexposedI_{unexposed}) which is 600 cases divided by 15,000 unexposed employees (0.04). The formula for PAR% is Itotal−IunexposedItotal×100\frac{I_{total} - I_{unexposed}}{I_{total}} \times 100. Plugging in the values gives 0.05−0.040.05×100=20%\frac{0.05 - 0.04}{0.05} \times 100 = 20\%.

Step-by-Step Solution

1
Calculate the total incidence of the outcome in the entire population (ItotalI_{total}).
Itotal=1,000 cases/20,000 total employees=0.05I_{total} = 1,000 \text{ cases} / 20,000 \text{ total employees} = 0.05 (or 5%).
PAR% requires knowing the overall incidence of the disease in the population to determine what proportion of all cases is attributable to the exposure.
2
Calculate the incidence of the outcome in the unexposed group (IunexposedI_{unexposed}).
Iunexposed=600 cases/15,000 unexposed employees=0.04I_{unexposed} = 600 \text{ cases} / 15,000 \text{ unexposed employees} = 0.04 (or 4%).
The unexposed incidence represents the background risk of disease that would exist even if the exposure were entirely eliminated.
3
Calculate the Population Attributable Risk Percent (PAR%) using the formula: Itotal−IunexposedItotal×100\frac{I_{total} - I_{unexposed}}{I_{total}} \times 100.
0.05−0.040.05×100=0.010.05×100=20%\frac{0.05 - 0.04}{0.05} \times 100 = \frac{0.01}{0.05} \times 100 = 20\%.
This calculation yields the percentage of all incident cases in the population that can be attributed to the exposure, representing the maximum potential benefit of eliminating the risk factor.

Key Concept

Population Attributable Risk Percent (PAR%) measures the proportion of disease in the total population that is attributable to the exposure. It is calculated as Itotal−IunexposedItotal×100\frac{I_{total} - I_{unexposed}}{I_{total}} \times 100.

Alternative Method

PAR% can also be calculated using the prevalence of the exposure in the population (PeP_{e}) and the Relative Risk (RRRR). Pe=5,00020,000=0.25P_{e} = \frac{5,000}{20,000} = 0.25. The exposed incidence is 4005,000=0.08\frac{400}{5,000} = 0.08, and unexposed is 60015,000=0.04\frac{600}{15,000} = 0.04, making RR=0.080.04=2.0RR = \frac{0.08}{0.04} = 2.0. Formula: PAR%=Pe×(RR−1)Pe×(RR−1)+1=0.25×(2−1)0.25×(2−1)+1=0.251.25=0.20PAR\% = \frac{P_{e} \times (RR - 1)}{P_{e} \times (RR - 1) + 1} = \frac{0.25 \times (2 - 1)}{0.25 \times (2 - 1) + 1} = \frac{0.25}{1.25} = 0.20, or 20%.
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Measures of Association and Impact

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