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Difficulty: HardMeasures of Disease Frequency and Association

A team of public health officials is investigating a recent outbreak of severe gastroenteritis in a suburban community. To determine the source of the outbreak, the investigators identify 100100 patients who were recently treated for the illness at the local hospital. They also recruit 150150 healthy individuals from the same community who did not develop any gastrointestinal symptoms. A detailed dietary history reveals that 6060 of the sick individuals and 3030 of the healthy individuals consumed food from a specific local food truck in the week preceding the outbreak.

Which of the following is the most appropriate measure of association for this investigation, and what is its calculated value?

  1. Odds ratio of 6.0Answer
  2. B
    Relative risk of 2.7
  3. C
    Odds ratio of 2.7
  4. D
    Relative risk of 6.0
  5. E
    Odds ratio of 0.17

Answer

Odds ratio of 6.0
The correct answer identifies that the described investigation is a case-control study and appropriately calculates the odds ratio. In this scenario, participants were selected based on whether they had the outcome of interest (gastroenteritis cases) or did not have the outcome (healthy controls). Their prior exposure to the food truck was then determined retrospectively. The appropriate measure of association for a case-control study is the odds ratio (OR), which compares the odds of exposure among cases to the odds of exposure among controls.

Using a standard 2x2 contingency table:
- Exposed cases (aa) = 6060
- Unexposed cases (cc) = 4040 (since 100−60=40100 - 60 = 40)
- Exposed controls (bb) = 3030
- Unexposed controls (dd) = 120120 (since 150−30=120150 - 30 = 120)

The odds ratio is calculated as:
OR=a/cb/d=60/4030/120=1.50.25=6.0OR = \frac{a/c}{b/d} = \frac{60/40}{30/120} = \frac{1.5}{0.25} = 6.0

Alternatively, it can be calculated using the cross-product ratio:
OR=a×db×c=60×12030×40=72001200=6.0OR = \frac{a \times d}{b \times c} = \frac{60 \times 120}{30 \times 40} = \frac{7200}{1200} = 6.0

Step-by-Step Solution

1
Determine the study design based on participant selection.
The investigators selected participants based on their disease status (100100 patients with gastroenteritis vs. 150150 healthy controls) and then looked retrospectively at their exposure (eating at the food truck). This is a case-control study.
Correctly identifying the study design is essential for selecting the appropriate epidemiological measure of association.
2
Select the appropriate measure of association.
For a case-control study, the appropriate measure is the odds ratio (OR). Relative risk (RR) cannot be calculated directly because the study does not measure the true incidence of disease in exposed and unexposed populations.
Case-control studies use odds ratios to estimate the association between an exposure and an outcome.
3
Extract data to form a 2x2 contingency table.
Cases (Ill): Total = 100100. Exposed (aa) = 6060, Unexposed (cc) = 100−60=40100 - 60 = 40.
Controls (Healthy): Total = 150150. Exposed (bb) = 3030, Unexposed (dd) = 150−30=120150 - 30 = 120.
Organizing the data is necessary to correctly apply the odds ratio formula.
4
Calculate the odds ratio.
OR=a/cb/d=60/4030/120=1.50.25=6.0OR = \frac{a/c}{b/d} = \frac{60/40}{30/120} = \frac{1.5}{0.25} = 6.0.
This yields the final numeric value of the statistical association.

Key Concept

Case-control studies select participants based on disease status (cases vs. controls) and assess prior exposures. The appropriate measure of association for these studies is the odds ratio (OR), calculated as the odds of exposure in cases divided by the odds of exposure in controls.
Estimated Time:1m 30s
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Topics

Measures of Disease Frequency and Association

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