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 use) and an outcome (like breast cancer).
Though they have the same goal, observational studies and randomized controlled trials differ in the way they are conducted and in the strength of the conclusions they reach.
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 medicines their health care providers prescribe. They report these activities to researchers.
There are two types of observational studies:
A prospective cohort study follows a large group of people forward in time. Some people will have a certain exposure (like alcohol use) and others will not. Researchers compare the different groups (for example, they might compare heavy drinkers, moderate drinkers, light drinkers and non-drinkers) to see which group is more likely to develop an outcome (like breast cancer).
A case-control study takes a different approach. Researchers identify two groups—one with people who already have an outcome (like breast cancer) and one with people who do not. They then compare the two groups to see if any exposure (like alcohol use) was more common in the history of one group compared to the other.
Randomized controlled trials use interventions (like an exercise class) to change some participants' behavior to see how it affects their health. Or, they may give certain treatments (like a new chemotherapy) to some participants to see how well it treats their breast cancer.
These studies are called randomized controlled trials because people are randomly assigned (as if by coin toss) to a certain behavior or treatment. For example, in a randomized controlled trial on exercise and breast cancer risk, 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.
Many behaviors, like smoking or heavy alcohol drinking, cannot be tested in this way because it would not be ethical to assign people to a behavior known to be harmful. In these cases, researchers must use observational studies.
A patient series is a health care provider’s observations of a group of patients who are given a certain treatment. There is no comparison group in a patient series. That is, all the patients are given a certain treatment and the outcomes of these patients are studied.
With no comparison group, it is hard to draw firm conclusions about the effectiveness of a treatment. If 10 women with breast cancer are given a new treatment, and two of them respond, how do we know if the new treatment is better than standard treatment? If we had a comparison group of 10 women with breast cancer who got standard treatment, we could compare their outcomes to the outcomes of the 10 women on the new treatment. If the no women in the comparison group responded to standard treatment, then the two women who responded to the new treatment would represent a success of the new treatment. If, however, two of the 10 women in the comparison group responded to standard treatment, then the new treatment is no better than the standard.
Thus, the lack of a comparison group makes it hard to draw conclusions from a patient series. However, data from a patient series can help form hypotheses that can be tested in other types of studies.
Each type of research study has its own strengths and weaknesses. These are important to be aware of when reviewing scientific evidence.
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 two important reasons. First, it can be hard for people to remember details about the past (especially when it comes to things like diet). 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 the problems of case-control studies because they gather information from people 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. These studies use the approach of a case-control study. However, they use data gathered as part of a cohort study, so they are less prone to bias than standard case-control studies. All things being equal, the strength of nested case-control data falls somewhere between that of standard case-control studies and cohort studies.
Randomized controlled trials are considered the gold standard for studying certain exposures, like cancer treatment. Similar to cohort studies, randomized trials follow people over time and are expensive to conduct. Because participants are randomly assigned to an intervention (like a new chemotherapy) or standard treatment, these studies are more likely to show the true link between an intervention and a health outcome (like survival).
Of course, the overall quality of a study is also important. For example, the results from a well designed case-control study can be more reliable than those from a poorly designed randomized trial.
To learn more about research study design, try a good, basic epidemiology textbook, such as Aschengrau A and Seage GR, III. Essentials of Epidemiology in Public Health, 2nd edition (Jones & Bartlett Publishers, 2008).
Animal studies add to our understanding of how and why some factors cause cancer in people. However, there are many differences between animals and people that make it hard to translate findings directly from one to the other. And, animal studies are designed differently than human studies. They often look at exposures in larger doses and for shorter durations than are suitable for people. Thus, animal studies can lay the groundwork for research in people, but in order to draw conclusions for human populations, we need human studies.
All data presented within the About Breast Cancer section of this website come from human studies unless otherwise noted.
Learn more about clinical trials, including the types of trials, benefits, and possible drawbacks.
How has having breast cancer changed your outlook?