Design of Experiments > Prospective study
What is a Prospective Study?
A prospective study (sometimes called a prospective cohort study) is a type of cohort study, or group study, where participants are enrolled into the study before they develop the disease or outcome in question. The opposite is a retrospective study, where researchers enroll people who already have the disease/condition. Prospective studies typically last a few years, with some (like the Framingham Heart Study) lasting for decades.
Study participants typically have to meet certain criteria to be involved in the study. For example, they may have to be of a certain age, profession, or race. Once the participants are enrolled, they are followed for a period of time to see who gets the outcome in question (and who doesn’t). Usually, the research is conducted with a goal in mind and participants are periodically checked for progress, using the same data collection methods and questions for each person in the study. Follow ups might include:
- Email questionnaires,
- Phone, internet, or in-person interviews,
- Physical exams,
- Imaging or laboratory tests.
Participants are followed for years and data is collected on the factors of interest, which might include:
- When the subject develops the condition,
- When they drop out of the study or become “lost,”
- When their exposure status changes,
- When they die.
Advantages and Disadvantages
- Disease outcomes and prevalence are east to calculate.
- Multiple disease and conditions can be studied at the same time.
- Researchers don’t have to deal with ethical issues like who receives which treatment (or none at all).
- All cohort studies can be expensive and time consuming.
- Confounding variables can be a larger problem with this type of study.
- Sample sizes are typically very large.
- Selection bias may be an issue.
Famous Prospective Study Examples
- Sir Richard Doll began a prospective cohort study in 1951, following 35,000 male physicians. Doll published his initial findings three years later, in 1954.
“I personally thought it was tarring of the roads. We knew that there were carcinogens in tar. It wasn’t long before it became clear that cigarette smoking may be to blame. I gave up smoking two-thirds of the way through that study” — Sir Richard Doll.
Follow ups lasted 50 years, and in 2004 Doll published “Mortality in relation to smoking: 50 years’ observations on male British doctors” in the British Medical Journal.
- The Framingham Heart Study is one example of a prospective cohort study; The researchers have, to date, studied three generations of Framingham residents in order to understand the causes of heart disease and stroke.
- The Nurses Health Studies, established in 1976, investigates risk factors for chronic diseases in women. The study has followed 280,000 male and female nurses to date.
- The Black Women’s Health Study seeks to understand why black women have higher rates of many illnesses like breast cancer and diabetes. The study has been following participants for more than 20 years.
- Caerphilly Heart Disease Study. 2500 men were recruited from a town in Wales to look at how environmental factors influence chronic diseases.
- Canadian Longitudinal Study on Aging / Étude longitudinale canadienne sur le vieillissement (CLSA-ÉLCV). “A large, national, long-term study of more than 50,000 men and women who were between the ages of 45 and 85 when recruited. These participants will be followed until 2033 or death. The aim of the CLSA is to find ways to help us live long and live well, and understand why some people age in healthy fashion while others do not.”
- The CARTaGENE Cohort, which consists of both biological samples and data on the health and lifestyles of more than 42,000 Quebecers between 40 and 69 years old.
- Cebu Longitudinal Health and Nutrition Survey.
- Rotterdam Study.
- Tsimane’ Amazonian Panel Study.
- The UK Biobank, studies serious and life-threatening illnesses like cancer, depression and dementia with 500,000 participants aged between 40 and 69 years. Started in 2006 and is ongoing.
American Journal of Epedemiology. Sir Richard Doll, 1912-2005. Retrieved July 22, 2016 from: http://aje.oxfordjournals.org/content/164/1/95.full.
Doll, R. et. al. Mortality in relation to smoking: 50 years’ observations on male British doctors. 2004. BMJ 2004;328:1519