Researchers in medicine, nursing, psychology, and some social science fields are found to group their subjects of study into cohorts before carrying out the required investigations on them. It is an epidemiological design that is used to easily arrive at conclusions on situations where they are difficult to come by.

Generally, a cohort is a group of people who share a common characteristic or experience within a particular period. For example, people who are exposed to a pollutant, dealing with drug addiction, have undergone a medical procedure, etc.

A cohort study is mostly used in the medical field, and significant research with this approach was commenced by Richard Doll and Austin Bradford-Hill in 1951. They wrote to all the registered doctors in the UK asking about their smoking habits, followed it up for decades, and eventually discovered the health risks of smoking.

What is a Cohort Study? 

A cohort study is a type of longitudinal study that recruits a group of individuals who share common characteristics over some time. It could be people with the same occupation, from the same race, demography, or even people suffering from the same disease.

Medical researchers use this approach to investigate diseases, find their causes, symptoms, and cure. In some cases, members of a cohort may be exposed to a specific risk factor or characteristic; then measure the outcome over some time.

A cohort study is an example of a panel study, which contains elements with the same characteristics. The study usually has 2 groups, the exposed and the non-exposed group.

For example, you may consider a group of cigarette smokers and those who don’t smoke to know who gets lung cancer and who doesn’t. This is a great way of knowing whether the lung cancer has anything to do with smoking.

There are three main types of cohort studies, namely, the ambidirectional cohort study, retrospective cohort study, and prospective cohort study. 

What is a Prospective Cohort Study?

A prospective cohort study is a type of cohort study whereby the researchers conceive and design the study, recruit subjects, and collect background data on all subjects before they start developing noteworthy outcomes. These subjects are then usually observed for a long period while recording interesting observations.  

The researcher will usually do a follow-up during this period through interviews, online forms, questionnaires, physical examinations, tests, etc. The subjects may be given some things as part of the investigation, which could involve recording the effect of these things on the subject.

In the typical sense, investigators do have a primary focus which will inform how the study is. For example, learning more about the causes of stomach ulcers.

What is a Retrospective Cohort Study? 

This cohort study groups subjects based on their exposure status and compare their incidence of disease. However, in this case, investigators go back in time to identify a group that was initially unexposed and study the incidence of their exposure.

They can use different methods to carry out these investigations including interviewing the cohort, consulting old newspapers & magazines, reports or journals that have talked about it in the past, etc. In essence, the investigators time travel to the past to identify a cohort that fits the required profile.

For example, we could have a retrospective cohort study whereby the investigators go back several decades to get the records of the factory workers from a manufacturing company. These workers could be investigated on how exposure to particular radiation from the machines affected them over the years.

What is an Ambidirectional Cohort Study? 

A cohort study that is ambidirectional is said to be both prospective and retrospective. This means that there are both prospective and retrospective phases of the study.

Although less common, the ambidirectional cohort study is conceptually consistent with and shares elements of both the prospective and retrospective studies. It includes the advantages, disadvantages, uses, applications, etc. of both cohort studies.

For example, the development of a new treatment for a particular medical issue may require studying the effects of this treatment on patients compared to the old one that was used in the past. A prospective cohort study will be carried out on new patients who will use this treatment for some time, while a retrospective cohort study will be carried out on patients who used the old treatment.

Here are 11 Key Differences between retrospective and prospective studies 

  • Definitions

Retrospective cohort study is a type of study whereby investigators design the study, recruit subjects, and collect background information of the subject after the outcome of interest has been developed while the prospective cohort study is an investigation carried out before the outcomes of interest have been developed. 

Although these 2 cohort studies group subjects according to their exposure status and compare their incidences, they are done at different times. Therefore, the outcome of a retrospective cohort study may be influenced by the investigator, which may not be the case for a prospective study.

This is mainly because the event has already occurred, and sometimes, there is no available data or the data sources are conflicting.

  • Examples

An example of a retrospective cohort study will be interviewing a cohort of people who are HIV positive, ask about their lifestyle choices and medical history to study the origins of the disease. A prospective cohort study example will be interviewing a cohort of people who are at high risk of getting infected with HIV for a couple of years to see if they will get infected.

We see that in the retrospective study example, the subjects have already been infected with HIV, while the subjects in the prospective study have not been infected. The goal of the first study is to determine the causes of HIV, and that of the second is to check whether the cohort will become HIV positive.

  • Data Collection

The data collected for the retrospective study may be gotten from medical reports from previous diagnoses, old articles, magazines, newspapers, etc. A prospective study, on the other hand, does not use old magazines and newspapers as a data collection tool.

Data is mostly collected by observing and interviewing the subject. The reason is clear from the fact that prospective study involves events that are yet to occur.

There are usually no old or existing reports of the event. The researchers are expected to perform independent research whose record may now be kept for the future retrospective study.


  • Data Analysis

In a retrospective study, the data may be analyzed immediately the investigator has access to it because there is enough data collected during a period to get meaningful assertions. In prospective study, on the other hand, needs some time before data analysis can take place.

Data analysis cannot take place until there is enough outcome to make an assertion. So, time must elapse before you can compare the incidence.

There is no condition to the amount of time that must elapse before the data is analyzed. Rather, it depends solely on the type of research that is being carried out, and at the researcher’s discretion.

  • Advantages

Retrospective studies are conducted on a small scale and do not require much time to complete, unlike the prospective surveys. In the medical context, they have the potential to address rare diseases.

For example, there are reports that Ebola has been eliminated from Nigeria and some other African countries where it was. Although not common at the moment, a retrospective study might be performed by investigating people who were cured of Evola in the past. 

In a prospective study, there is usually room for fewer biases compared to a retrospective study. Also, a prospective study has more useful applications in modern medicine.

  • Disadvantages

A prospective cohort study is usually very expensive compared to a retrospective cohort study. This is usually because there are no available material or record that contains relevant data that shows the required outcome.

Therefore, all the data will have to be collected by the researcher, and therefore incurring more cost than the retrospective study. Although there is usually data to choose from when dealing with the retrospective study, sometimes, this data may be unavailable, making it difficult to perform proper investigation. 

Since this is something that has happened in the past, it might not be easy to gather this data again. In some cases, this data may be corrupted and inaccurate as it is being passed from one source to another. 

  • Use Cases

Retrospective cohort study is mostly conducted to build on existing research or discovery while prospective cohort study helps to make discoveries. In some cases, we could say prospective studies are used to discover new theories.

When conducting a study to discover the reason why an event occurred we use a retrospective study. But if we want to discover if it will occur, we settle for a prospective study.

They can be used for the same purpose, but through different approaches. For example, a researcher who wants to find out what the symptoms of malaria are can use both retrospective and prospective study.

For the retrospective study, people who have had malaria in the past will be investigated to discover the symptoms they had and the common ones will be mapped. In the case of a prospective study, people who are not vaccinated against malaria will be studied to see if they will eventually get the disease.

If they get the disease, a report of the symptoms noticed during the study will be reported.

  • Sample size

Retrospective study is conducted on a smaller scale, and as such make use of a small sample size. Two sample sizes may be taken and compared together, but they are usually small samples.

A prospective study, on the other hand, is conducted on a larger scale compared to a retrospective study. This may likely be because it may be difficult to find subjects for the cohort because some of them may be dead or unwilling to share their past.

For example, an HIV positive patient may be uncomfortable being part of a cohort because he/she still regret not using protection. Or especially if it is a case of rape and may bring back memories of the rape.

  • Cost

The cost of performing a retrospective survey is lesser than a prospective survey. Investigators tend to spend less on data collection and may even take lesser years of observation.

In most investigations, data collection is known to cost more. Therefore, less data collection will result in less cost.

Since the investigator is starting afresh when performing a prospective study, it requires more data collection and will automatically cost more.

  • Time

The time taken when performing a prospective study is usually longer compared to the retrospective study. In a retrospective study, there is already available data for the researcher to extract from, thereby reducing the time spent on data collection.

Therefore, the only thing that is required is to make longitudinal observations and perform data analysis. A prospective study, on the other hand, involves waiting for the outcome of interest to showcase before analysis can be done. 

Some time will be spent gathering subjects, collecting background data, and waiting for enough data to be gathered before analysis will eventually take place.

  • Cohort

The cohort required for a prospective study is usually a set of people who have not experienced the event before, but are just prone to it. A retrospective study requires a cohort that has experienced the event in the past.

There are clear differences in the kind of cohort required for retrospective and prospective study. For example, a medical researcher is investigating the causes of HIV in humans.

A retrospective approach will recruit subjects who are HIV positive, while a prospective approach will recruit people who are HIV negative but are at risk of contracting the disease. This may include people who have sex without protection, share clipper with a 3rd party, have an HIV positive partner, etc.

Similarities Between Prospective and Retrospective Studies 

Prospective and retrospective studies see both examples of longitudinal studies. The process involved in these studies is almost the same, except for the fact that prospective studies are done before the outcome and retrospective survey is done after.

Also, the data used for prospective and retrospective studies can be collected using similar tools. For example, interviews, online forms, questionnaires, diagnosis, etc. 

This includes the first data collected before the initial outcome (i.e. in the case of prospective data) and the subsequent data collected during the period of observation. Although there are some differences in the process of data collection, they do interact at some point.

After collecting the data, the next step is data analysis, which is very similar in prospective and retrospective studies. It is noteworthy that the data analysis process in a prospective study is retrospective.

The data collected will take some time before it is analyzed because data analysis cannot take place until enough events or outcomes have occurred. This is where the analysis becomes retrospective.

Relationship Between Case-Control, Prospective and Retrospective Studies 

A case-control study is a kind of study designed to determine if an event is associated with an outcome. For example, if having unprotected sex is associated with contracting HIV.

There are different steps involved in case-control, but the first thing is to identify the cases (i.e. infected cohort) and the control (uninfected cohort). After then, you should look back in time to know the members of the case who were exposed to unprotected sex and compare the frequency of the exposure in the case-cohort to the control cohort.

Also known as a case-referent study, the process is said to be retrospective because it starts with an outcome, before tracing it back to the exposures. It is also a type of epidemiological study and is often mistaken for a cohort study.

By relationships between prospective and retrospective studies, case-control is more of a retrospective study compared to a prospective study. Therefore, there is not much relationship between prospective study and case-control.

Case-control studies are relatively inexpensive and frequently used because it can be carried out by small teams and individual researchers. Compared to the prospective cohort study, they are cheaper and tend to take less time.


A cohort study is an important aspect of epidemiological research, understanding risk factors and also providing a possible treatment for diseases in various participants all over the world. Groundbreaking medical discoveries, vaccines, antidotes, medications, etc are arrived at through cohort study.

Although useful in other aspects, it is mostly used in the medical field to make investigations that uncover new diseases, symptoms, and cures. This approach does bring with it some challenges that are often related to sample size complexity, longevity, etc.

However, if carefully planned and implemented, a cohort study can make valuable contributions to different sectors of the economy. To do this, one should have a proper understanding of the different types of a cohort study and its relationship with other important study techniques.

  • busayo.longe
  • on 12 min read


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