There are two main types of epidemiologic research studies: observational studies and randomized controlled trials. In both types of studies, the goal is to give information that helps support or disprove an idea (called a hypothesis). A hypothesis for a study might be a link between an exposure (like alcohol consumption) and an outcome (like breast cancer). Though they have the same goal, observational studies and randomized controlled trials differ in the way that they are conducted and in the strength of the conclusions they reach.
Observational Studies
In observational studies, the study participants live their daily lives as they choose: they exercise when they want, eat what they like and take the medications or get the medical procedures their health care providers prescribe. They then report their activities to researchers.
There are two types of observational studies: prospective cohort studies and case-control studies. A prospective cohort study follows a large group of people forward in time. Some of these people have a certain characteristic (such as alcohol use) and some do not. Researchers compare the groups who differ on the characteristic (for instance, they might compare heavy drinkers, moderate drinkers, light drinkers and non-drinkers)—to see who develops the outcome of interest. A case-control study takes a very different approach. Researchers identify two groups—one with people who already have the outcome of interest (such as breast cancer) and one with people who do not. They then compare the two groups to see if any characteristic was more common in the history of one group compared to the other.
Randomized Controlled Trials
In contrast to observational studies (prospective cohort or case-control), randomized controlled trials use interventions (like an exercise class) to change some participants' behavior or give a certain therapy to see how it affects their health. These studies are called randomized controlled trials because the participants are randomly assigned (as if by coin toss) to a certain behavior or treatment. For example, to do a randomized controlled trial on exercise and breast cancer, researchers might randomly assign half of the participants to walk 10 minutes a day, and half to walk two hours a day. They would then see who was more likely to develop breast cancer: those who walked 10 minutes a day or those who walked two hours a day. Of course, many behaviors, like smoking or heavy alcohol drinking, cannot be tested in this way because participants cannot ethically be assigned to a behavior that is known to be harmful. In these cases, researchers are limited to the use of observational studies.
Different studies (prospective cohort, case-control, randomized controlled trial) have different strengths and weaknesses, and these are important to be aware of when reviewing scientific evidence. If you are interested in learning more about these study designs, try a good, basic textbook in epidemiology, such as Epidemiology in Medicine by C. H. Hennekens and J. E. Buring (Lippincott Williams & Wilkins, 1987).
The Different Research Studies—A Brief Tour of Strengths and Weaknesses
Each type of research study has its own set of strengths and weaknesses. Case-control studies, while easy and fairly inexpensive to conduct, can be prone to inaccuracy because they use information about the past. This is a concern for a couple of reasons. First, it can be hard for people to remember specifics about the past, especially when it comes to things like diet. And second, memories can be biased (or influenced) by the fact that the information is gathered after an event (like the diagnosis of breast cancer). Prospective cohort studies avoid many of these problems because they gather information from study participants over time and before the events being studied happen. But, compared to case-control studies, they are expensive to do.
A nested case-control study is an offshoot of a prospective cohort study. This type of study uses the approach of a case-control study. However, it uses data gathered as part of a cohort study, so it is less prone to bias than a standard case-control study. All things being equal, the strength of nested case-control data falls somewhere between that of standard case-control studies and cohort studies.
Finally, there are randomized controlled trials, considered the gold standard for studying certain exposures, like cancer therapies. Similar to cohort studies, randomized trials follow people over time and are expensive to conduct. Because participants are randomly assigned to get an intervention (like radiation therapy) or not, it is thought that these studies are more likely to show the true link between an intervention and a health outcome (like overall survival).
Of course, the overall quality of a study is also important. The results from a well-designed case-control study, for example, can be more reliable than those from a poorly designed randomized trial.
Patient Series
Another type of study that is sometimes used in health research is a “patient series”. A patient series is a health care provider’s observations of a group of patients who are given a certain type of therapy. There is no comparison group in a patient series. That is, all the patients are given some sort of treatment, and the outcomes of the patients are studied. With no comparison group, as there would be in a randomized controlled trial or an observational study, it is very hard to draw firm conclusions about the effect of the treatment on the outcome being studied. If 10 women with advanced breast cancer are given a new, experimental treatment, and eight of them do not respond, how do we know if the new treatment is good or bad? If we had a comparison group of 10 women with advanced breast cancer who got standard therapy, we could compare outcomes. If all the women in the comparison group did not respond to standard therapy, then the two who responded to the new treatment would represent a success of the new treatment. If, on the other hand, two of the 10 women in the comparison group responded to the standard therapy, then the new treatment is no better than the standard therapy. Thus, without a comparison group, it is hard to draw conclusions from a patient series. However, data from a patient series can make hypotheses that can be tested in more strictly designed studies.
Other types of research studies
Animal studies add to our understanding of how and why certain factors cause cancer in humans and can provide early evidence for research in human populations. They allow us to study exposures that are difficult to study in humans and help identify potential new therapies for treating cancer. While there are many similarities between animals and humans, there are many differences that make it difficult to translate findings directly from one to the other. Moreover, animal studies are designed differently than human studies and often look at exposures or therapies in larger doses and for shorter durations than are suitable for humans [1]. In this way, animal studies can lay the groundwork for human research, but in order to draw conclusions for human populations, we need human studies. All data presented on this website come from human studies unless otherwise noted.
References
1. Ames BN, Gold LS. Chemical carcinogenesis: too many rodent carcinogens. Proc Natl Acad Sci U S A. 87(19):7772-6, 1990.
Updated 03/13/09
