A prospective study (sometimes called a prospective cohort study) is a type of cohort study or group study, in which participants enroll in 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 usually last a few years, and some (such as the Framingham Heart Study) last decades. Study participants generally have to meet certain criteria to participate in the study. For example, they may have to be a certain age, profession, or race. Once participants are enrolled, they are followed for a period of time to see who gets the result in question (and who doesn't).

Objectives of the Prospective Study

The research is usually conducted with a goal in mind and the progress of the participants is checked periodically, using the same data collection methods and questions for each person in the study. Follow-ups can include:

  • Email questionnaires,
  • Telephone, internet or in-person interviews,
  • Physical exams
  • Imaging or laboratory tests.
  • Results

Participants are followed for years and data is collected on factors of interest, which may include:

  • The subject develops the condition,
  • They leave the study or are "lost",
  • Their exposure status changes,
  • When they die.

Prospective and retrospective studies

Prospective Studies

The study generally involves taking a cohort of subjects and observing them for an extended period. The outcome of interest must be common; otherwise, the number of observed results will be too small to be statistically significant (indistinguishable from those that may have arisen by chance). Every effort should be made to avoid sources of bias, such as loss of people to follow-up during the study. Prospective studies tend to have fewer potential sources of bias and confusion than retrospective studies.

Retrospective Studies

A retrospective study looks back and examines exposures to suspected risk or protective factors in relation to a result that is established at the beginning of the study. Many valuable case-control studies, such as Lane and Claypon's 1926 investigation of breast cancer risk factors, were retrospective investigations. Most sources of error due to confounding and biases are more common in retrospective than prospective studies. For this reason, retrospective research is often criticized.

However, if the outcome of interest is rare, the size of the prospective investigation required to estimate the relative risk is often too large to be feasible. In retrospective studies, the odds ratio provides an estimate of the relative risk. Special care must be taken to avoid sources of bias and confusion in retrospective studies. Prospective investigation is required to make accurate estimates of the incidence of an outcome or the relative risk of an outcome based on exposure.

Examples of famous prospective studies

Sir Richard Doll started a prospective cohort study in 1951, following 35,000 male physicians. Doll published his initial findings three years later, in 1954. “Personally, I thought the roads were breaking down. We knew there were carcinogens in the tar. It wasn't long before it became clear that smoking may be the culprit. I quit smoking two-thirds of the way through that studio. ”- Sir Richard Doll. The follow-ups lasted 50 years, and in 2004 Doll published “Smoking-Related Mortality: 50-Year Observations of Male British Doctors” in the British Medical Journal.

The Framingham Heart Study is an example of a prospective cohort study. To date, researchers have studied three generations of Framingham residents to understand the causes of heart disease and stroke.

The Nurses Health Studies, established in 1976, investigate risk factors for chronic diseases in women. This 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 diseases such as breast cancer and diabetes. This study has followed the participants for more than 20 years.

Internet studies

Other examples

Caerphilly Heart Disease Study. 2,500 men were recruited from a town in Wales to observe how environmental factors influence chronic disease.

Canadian Longitudinal Study on Aging / Étude longitudinalinale canadienne sur le vieillissement (CLSA-ÉLCV). “A large long-term national study of more than 50,000 men and women who were between 45 and 85 years old when they were recruited. These participants will be followed until 2033 or until death. CLSA's goal is to find ways that help us live long and live well, and understand why some people age healthily and others don't. "

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 Amazon Panel Study. The UK Biobank studies serious and life-threatening illnesses such as cancer, depression and dementia with 500,000 participants aged 40-69. It started in 2006 and is ongoing.

Cohort Studies

Cohort studies are usually, but not exclusively, prospective, the reverse is true for case-control studies. The following notes relate the cohort to the case-control studies:

  • The result is measured after exposure
  • Produces real incidence rates and relative risks
  • You can discover unexpected associations with the result.
  • Best for common results
  • Is expensive
  • Requires large numbers
  • Takes a long time to complete
  • Prone to attrition bias (compensate by using person time methods)
  • Prone to method change bias over time
  • Related methods are risk (prospective), relative risk meta-analysis, risk difference meta-analysis and proportions

Advantages and disadvantages


Multiple variables can be studied at the same time. Researchers don't have to grapple with ethical questions like who gets what treatment (or none).


All cohort studies can be expensive and time consuming.

Confounding variables can be a major problem with this type of study.

Sample sizes are usually very large.

Selection bias can be a problem.

Strengths of a cohort study

Temporality: given that at the time of entering the cohort study, the individuals do not have a result, the temporality between the exposure and the result is well defined

A cohort study helps us study multiple outcomes on the same exposure. For example, if we follow patients with hypercholesterolemia, we can study the incidence of melasma or psoriasis in them. Therefore, there is one exposure (hypercholesterolemia) and multiple outcomes (melasma and psoriasis).

However, we have to make sure that none of the individuals have any of the baseline results. If exposure is rare, then a cohort design is an efficient method to study the relationship between exposure and outcomes. In general, it is said that a cohort design may not be efficient for rare outcomes (a case-control design is preferred). For example, melanoma is not a common condition in India. Therefore, if we follow people to study the incidence of melanoma, it may not be effective. In a prospective cohort study, the exposure variable, other variables, and outcomes can be measured with greater precision. T

his is important to maintain consistency in metering exposures and results. This is also useful for exposures that may require subjective assessment or recall on the part of the patient. For example, dietary history, smoking history, or alcoholism history, etc. This can help reduce bias in the exposure metering. A retrospective cohort study can be completed quickly and is relatively inexpensive compared to a prospective cohort study. However, it also has other strengths of the prospective cohort study.

Limitations of a cohort study

A major limitation of a prospective cohort design is that it is time consuming and expensive. For example, if we have to study the incidence of cardiovascular patients in psoriasis patients, we may have to follow them for many years before the result occurs. In a retrospective cohort study, exposure and outcome variables are collected before the study starts. Therefore, the measurements may not be very precise or in accordance with our requirements. Also, some of the exposures may have been assessed differently for various members of the cohort. As discussed above, cohort studies may not be very efficient for rare outcomes, except under some conditions.

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Prospective Studies

Prospective Studies

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