Cohort studies, case control studies and randomized controlled trials

When people read about a research study, they may not pay attention to how the study was designed. But to understand the quality of the findings, it's important to know a bit about study design.

According to the widely-accepted hierarchy of evidence, the most reliable evidence comes from systematic reviews (see Infocus, Summer 2004), followed by evidence from randomized controlled clinical trials, cohort studies and then case control studies.

Research studies typically fall into one of two main categories:

Observational studies

Here researchers observe the effect of a risk factor, diagnostic test or treatment without trying to influence what happens. Such studies are usually "retrospective" — the data are based on events that have already happened. Most workplace health research falls into this category.

Cohort study: For research purposes, a cohort is any group of people who are linked in some way and followed over time. Researchers observe what happens to one group that's been exposed to a particular variable — for example, the effect of company downsizing on the health of office workers. This group is then compared to a similar group that hasn't been exposed to the variable.

Case control study: Here researchers use existing records to identify people with a certain health problem (“cases”) and a similar group without the problem (“controls”). Example: To learn whether a certain drug causes birth defects, one might collect data about children with defects (cases) and about those without defects (controls). The data are compared to see whether cases are more likely than controls to have mothers who took the drug during pregnancy.

Some strengths of observational studies

This may be the only way researchers can explore certain questions. For example, it would be unethical to design a randomized controlled trial (see below) deliberately exposing workers to a potentially harmful situation.

But...

The results of observational studies are, by their nature, open to dispute. Example: A cohort study might find that people who meditated regularly were less prone to heart disease than those who didn't. But the link may be explained by the fact that people who meditate also exercise more and follow healthier diets.

Experimental studies

Here researchers introduce an intervention and study the effects. Experimental studies are usually randomized, meaning the subjects are grouped by chance. While not all controlled studies are randomized, all randomized trials are controlled.

Randomized Controlled Trial (RCT)

Eligible people are randomly assigned to two or more groups. One group receives the intervention (such as a new drug) while the control group receives nothing or an inactive placebo. The researchers then study what happens to people in each group. Any difference in outcomes can then be linked to the intervention.

Controlled Clinical Trial (CCT)

This is similar to an RCT, except that subjects are not randomly assigned to the treatment or control groups. This increases the chance for “bias”–that is, that people with similar qualities ended up in each of the groups which could influence the final results.

Some strengths of experimental studies

The RCT is still considered the “gold standard” for producing reliable evidence because little is left to chance.

But...

There's a growing realization that such research is not perfect, and that many questions simply can't be studied using this approach. Such research is time-consuming and expensive — it may take years before results are available.
 

Source: At Work, Issue 42, Fall 2005: Institute for Work & Health, Toronto