Introduction
Consider a hypothetical island population of 6,647 residents in which 13 people have been diagnosed with a rare neurological problem. Some of the residents think that the problem was caused by eating apples that had been sprayed with a particular pesticide used in one of the islands four orchards. We could ask all of the residents if they had eaten apples to assess their exposure, but assessing exposure in this way is complicated because all the grocery stores acquire apples from all four orchards, but the apples from all four orchards are McIntosh apples, and they all get mixed together. In addition, the neurological problem probably requires a certain threshold internal dose. A given individual's internal dose would depend on whether they ate apples, which orchard the apples came from, how many apples they ate, whether they had thoroughly washed the apples before eating them, whether the apples were peeled before consumption, the body weight of the individual, and their age. This makes it difficult to obtain the exposure information we need. An alternative is to take blood samples from all 6,647 residents and send them to a lab to have the pesticide concentration in their blood measured. However, it each sample would cost $50 to analyze making the total cost $332,350, and the health department can only spare about $4,000 on this investigation. With $4,000 we could take a random sample of 80 residents, but the disease is so rare that there would likely be few, if any, diseased people in our sample, and there might be few apple eaters among the 80 people we sampled. The problem here is two-fold; the disease is rare and it is difficult to get exposure data. A case-control design offers a possible way to circumvent these obstacles and test a hypothesis in this situation.
Essential Questions
- What are the different strategies for investigating the causes or sources of health outcomes?
- How do we choose the best approach to study a particular health problem?
- What are the strengths and limitations of different study designs?
- How can we study rare health outcomes?
Learning Objectives
After completing this section, you will be able to:
- Describe the design features and the advantages and weaknesses of case-control studies
- Explain how different study designs can be applied to the same hypothesis to provide different and complementary information.
- Explain how the sampling strategy of case-control studies differs from that of cohort studies and clinical trials
- Compute and interpret odds ratios in case-control studies