How is a prospective study different from a retrospective study?

Randomized controlled trials may be a key component of medical studies, but they can't provide all the answers medical professionals need. Because they involve patients who already have a condition, randomized controlled trials perform best in measuring the effectiveness of a treatment.

However, they cannot provide medical professionals with advice on preventing diseases to begin with. Learning more about the causes of diseases requires other types of studies.

Observational studies that point to potential causes of a disease can lead medical professionals and educators to steer patients away from harmful behaviors, locations, and other factors.

A prospective study, also known as a prospective cohort study, follows a group of people over a long period before they develop diseases. These studies have proved effective in determining environmental and behavioral factors that contribute to a variety of diseases, including heart disease, cancer, and many more.

A prospective study examines a group of people similar in some way before they have developed a disease or outcome. They are studied over an extended time to determine what factors might contribute to a certain disease or group of diseases.

Participants are selected based on a variety of factors, depending on what the study author believes will provide the best source of information, such as:

• Age

• Race

• Gender

• Geography

• Socioeconomic status

• Profession

For example, if the author were examining causes of prostate cancer, only men would be included in the study, whereas an examination of cervical cancer causes would focus on women.

Study participants provide a medical history at the beginning of the study and are examined. Throughout the study, they are periodically examined and asked about lifestyle and environmental issues.

As the study progresses, researchers build a large database of information to determine what factors might have contributed to some participants getting the disease but not others. For instance, researchers would have gathered data about eating habits, smoking or drinking habits, whether the subjects live near a potentially toxic source, and more.

What is the difference between prospective and longitudinal studies?

Prospective studies observe subjects over time, starting from the present and moving forward. 

Longitudinal studies track people over an extended period, often without a predefined start point, to analyze changes and relationships over time.

Prospective and retrospective studies look at causes from opposite directions. A retrospective study begins with patients who have developed a disease and looks at their history to determine what factors might have contributed to the illness. For example, a retrospective study of lung cancer patients might show that 80 percent had been cigarette smokers.

A retrospective study relies on the memory and honesty of study participants to gain an accurate view of their history. In the lung cancer example, researchers would need to determine when participants started smoking cigarettes, how heavily they smoked over various periods, and if there were other contributing factors.

A prospective study gathers the data as the study progresses, making it more likely to be accurate and giving a fuller picture of risk factors.

Can a study be both prospective and retrospective?

A study cannot be both prospective and retrospective because the two types of study begin with different sources (people before the onset of a disease or condition versus those with the disease or condition) and look in opposite directions.

Participants in a prospective study could become participants in a retrospective study once they have developed a disease or condition.

The majority of prospective studies are observational (prospective cohort studies), but researchers could also design a study (randomized controlled trial) with more structure and follow the outcome.

Prospective cohort studies

A prospective cohort study would gather participants and collect data from questionnaires or examinations without trying to control how the subjects lived.

Participants would in effect divide themselves into subgroups based on the decisions they made or how their health changed over time. Of course, the primary differentiator is whether or not they end up acquiring the disease.

Randomized controlled trials

Researchers could gain control over their subjects' actions by establishing a randomized controlled trial where participants follow a set of rules, and outcomes are judged on the results of the different groups.

For example, researchers could set up a study in which one group eats a vegan diet, a second group has a high-protein diet, and a third follows a high-carb diet.

Such studies could take place over a shorter timeframe than a prospective cohort study.

Is a prospective study a primary study?

A prospective study is a primary study because the authors observe participants and gather original material from them.

There are several advantages of using the prospective study approach in medical research, including:

• Quick disease detection: Outcomes are easy to obtain because participants will have regular exams so researchers can quickly determine when a disease is present.

• Broad risk factors: Researchers examine a wide range of possible risk factors.

• Multiple diseases simultaneously: Multiple diseases or outcomes can be studied in the same group.

• Ethical decisions: Medical professionals don't have to make ethical decisions about treatment options.

There are also disadvantages to the prospective study approach, including:

• Resources: Ideally, prospective studies need a large group of participants, so they can be expensive and time-consuming.

• Participant loss: The long time commitment could lead to the loss of participants, either through dropping out or death by other causes.

• Selection bias: Researchers must be careful to avoid bias when selecting participants.

• Confounding factors: Researchers need to be open to the possibility of confounding factors as they can affect outcomes.

While prospective studies offer great opportunities to determine the causes of diseases, they are not designed to measure the effectiveness of treatment options.

The results might also be limited to the studied population. For instance, studying breast cancer causes among Scandinavian women might not have any significance for the cause of breast cancer among African women or Latin women.

The Framingham Heart Study

The Framingham Heart Study is probably the most famous prospective cohort study. Since 1948, for over 70 years the Framingham Heart Study has followed an initial cohort of 5,209 men and women from Framingham, Massachusetts, and their ensuing generations.

The study has been a leading source of knowledge about the causes of heart disease and stroke.

Dr. Richard Doll

The late Dr. Richard Doll launched a prospective study of British doctors and the link between smoking and lung cancer in 1951.

Doll and his researchers published their first findings in 1954 and continued to follow the doctors for another 50 years.

Nurses’ Health Study

Influenced by Doll's work, researchers in the United States established the Nurses' Health Study in 1976.

They have studied more than 280,000 female and male nurses over the decades to examine several health issues.

Black Women’s Health Study 

Boston University, which now coordinates the Framingham Heart Study, created the Black Women's Health Study in 1995. The study was established to determine why black women suffer higher incidence and earlier onset of such diseases as hypertension, breast cancer, diabetes, stroke, and lupus.