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Difficulty: MediumMeasures of Disease Frequency and Association

A clinical research team investigates the potential link between maternal exposure to agricultural pesticides and the occurrence of neural tube defects. They identify 40 infants born with neural tube defects and 120 healthy infants born at the same regional hospital over a two-year period. The researchers subsequently review maternal health records and conduct structured interviews regarding residential history during early pregnancy. The data indicate that 30 mothers of the affected infants and 30 mothers of the healthy infants lived within one mile of active agricultural zones where pesticides were regularly applied. Which of the following values best estimates the association between maternal pesticide exposure and neural tube defects in this study?

  1. A
    0.11
  2. B
    0.40
  3. C
    3.0
  4. D
    5.0
  5. 9.0Answer

Answer

9.0
The study described is a case-control study because participants were identified based on their disease status (neural tube defects vs. healthy) rather than their exposure status. In a case-control study, the baseline incidence of the disease in the population is not known because the researchers artificially set the number of cases and controls. Therefore, relative risk cannot be directly calculated. The appropriate measure of association is the exposure odds ratio, which compares the odds of exposure among cases to the odds of exposure among controls. This is calculated as (a/c)/(b/d)(a/c) / (b/d). In this scenario, the odds of pesticide exposure among cases is 30/10=330/10 = 3, and the odds of exposure among controls is 30/90=1/330/90 = 1/3. Dividing these odds yields an odds ratio of 3/(1/3)=9.03 / (1/3) = 9.0.

Step-by-Step Solution

1
Identify the study design.
The study selected participants based on outcome (neural tube defects vs. healthy), making it a case-control study.
Study design dictates which statistical measure of association can be used.
2
Determine the appropriate measure of association.
Odds ratio (OR).
Relative risk and absolute risk reduction cannot be calculated in case-control studies because true incidence is unknown.
3
Extract the data to determine exposure odds for cases.
Among the 40 cases, 30 were exposed and 10 were unexposed. The odds of exposure = 30/10=330 / 10 = 3.
Needed for the numerator of the OR equation.
4
Extract the data to determine exposure odds for controls.
Among the 120 controls, 30 were exposed and 90 were unexposed. The odds of exposure = 30/90=1/330 / 90 = 1/3.
Needed for the denominator of the OR equation.
5
Calculate the odds ratio.
OR=31/3=9.0OR = \frac{3}{1/3} = 9.0.
This yields the final measure of association.

Key Concept

The odds ratio is the appropriate measure of association for a case-control study and is calculated by dividing the odds of exposure among cases by the odds of exposure among controls.
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Topics

Measures of Disease Frequency and Association

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