What researchers mean by... case control study

Case control studies start with an outcome (such as a disease) and work backwards to find exposures that may be linked to it.

Let’s say your mother was recently diagnosed with breast cancer and so, too, was her best friend. The two worked together for 30 years at the town’s food canning plant. You wonder if something in the workplace was the cause of their cancer.

Researchers can help find answers to this type of question using a case control study. This study design helps determine if a previous exposure is linked to a current condition, such as having a disease.

A case control study compares people who already have a condition or disease (these are the cases) with people who do not have the condition or disease but are otherwise similar (these are the controls). It then looks back to see if an exposure to something in particular (e.g. at work, in the environment, lifestyle) was more likely in the group with the condition than in the group without.

Not all studies with cases and controls are case control studies. Some studies start with a group of people with a known exposure and a comparison group (the control group) without the exposure and follow them forward to see what happens. This is the case with some cohort studies.

Case control studies are always retrospective; they always look back. The outcomes are always known—the cases do have the condition and the controls do not—and the researchers trace backwards to identify possible exposures or factors that may have contributed to the condition.

Case control study in action

Let’s take our example of breast cancer and work to show how a case control study might provide some answers. The researchers begin by turning to the regional cancer treatment centre to find women within the town and the surrounding area who developed a new case of breast cancer during a six-year period and are willing to participate in the study. The researchers identify 1,000 women, the cases.

The researchers then select controls. With computer-generated phone numbers, homes are randomly called to find women in the region without breast cancer of about the same age who are willing to take part in the study. They find 1,150 women, the controls.

Both cases and controls are asked about their personal, lifestyle and reproductive pasts, including information about factors known to be associated with breast cancer (e.g. body mass index, drinking, smoking, menstrual and menopause history, use of hormone replacement therapy, birth control, family history). They are also asked about the jobs they’ve had over the years and for how long. The researchers take this job information to code occupation, industry and exposure, allowing them to figure out likely exposures to cancer-causing materials and endocrine disruptors (i.e. chemicals that interfere with the hormone system).

By comparing the two groups, the researchers find that, taking the other risk factors into account, the women with breast cancer are more likely to have worked in certain occupations, including food canning. Although the study cannot say that your mother and her best friend’s breast cancer was caused by work—case control studies cannot show causation—it does indicate that their breast cancer may be linked to their work.

Case control studies have a number of drawbacks. They cannot show causation, as mentioned; nor can they provide information on incidence (e.g. what percentage of people have a condition). As well, the information collected can be faulty or incomplete because it depends on people accurately and truthfully recalling their past.

Nonetheless, case control studies are relatively quick, inexpensive and easy. Thus, they are often used to conduct preliminary investigations of suspected risk factors. If a link is found, a more costly study that starts with a group of people and follows them forward may be justified.

Note: The example used here is based on a real study published in the November 2012 issue of Environmental Health (vol. 11, no. 87, doi:10.1186/1476-069X-11-87). This case control study found an association between breast cancer among women and working in jobs with potentially high exposures to carcinogens and chemicals that interfere with the hormone system, including jobs in agriculture, automotive plastics manufacturing, food canning, metalworking, and bars and casinos.

Source: At Work, Issue 72, Spring 2013: Institute for Work & Health, Toronto