Question

If you wanted to conduct a case-control study to discover whether there was a relationship between Parkinson’s disease and having worked in a high stress occupation…

A. How might you go about selecting the cases and the controls for this study?

B. Assume that after you have selected the cases and the control you have a total of 1,000 cases and 2,000 controls. You find that among the cases 200 worked in a stressful occupation and among the controls 390 worked in a stressful occupation. What would be the odds ratio describing the relationship between working in a high stress occupation and having Parkinson’s disease?

C. Interpret the meaning of this odds ratio.

Answer 1

(a).

The purpose of the control group in a case-control study id to provide information on the exposure distribution in the population that gave rise to the cases.

Thus while selecting the cases I will prefer newly diagnosed cases, because they tend to be more motivated to participate, and may remember relevant exposures more accurately, and because it avoids complicating factors related to selection of longer duration (i.e., prevalent) cases. However, it is might be difficult to have an adequate sample size if only recent cases are enrolled. Thus cases other than recent cases could also be selected depending on the sample size required.

Sources of cases:

• Patient rosters at medical facilities
• Death certificates
• Disease registries
• Cross-sectional surveys (e.g., NHANES, the National Health and Nutrition Examination Survey)

it is important to think of a case-control study as being nested within some sort of a cohort, i.e., a source population that produced the cases that were identified and enrolled. According to this there are two key principles that I will follow for selecting controls:

1. The comparison group ("controls") should be representative of the source population that produced the cases.
2. The controls must be sampled in a way that is independent of the exposure, meaning that their selection should not be more (or less) likely if they have the exposure of interest.

both the above principles must be adhered to avoid any selection bias that might result due to improper sampling. Depending on the number of sample I could get or require, I will follow the above method.

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(b).

total number of cases = 1000

number of cases worked in stressful environment = 200

number of cases that didn't work in stressful environment = 800

odds = 200/800 = 0.25

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(c).

The odds ratio represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.

In above case odds are 1/4 of exposure in cases, which means there is 1/(1+4) that is 20% chances of having had exposure in case parkinson's disease.

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