What is a cohort study?

You’ve probably heard of people filling out medical questionnaires. This is a type of cohort study. 

A cohort study is a scientist's way of discovering what may have led to specific diseases and conditions so they know how to handle them.

You probably have a friend or neighbor who has lived in your neighborhood for some time. This means you belong to the same cohort. 

A cohort is simply a group of people with one or more common characteristics.

A cohort study focuses on monitoring groups of people over time to obtain data. While anyone can do a cohort study, researchers and scientists use them the most. 

Whenever a new disease emerges, scientists may do a cohort study while making the vaccine to examine the different reactions various groups of people will have toward it.

A cohort study is very similar to a case study. The major difference is that a cohort study compares the outcomes from certain experiments. 

In contrast, a case study compares people who share a specific outcome from a specific exposure. For example, if a work environment tries a new product and employees fall ill, researchers can use a case study to determine the exact cause of the illness.

The ideal type of cohort study depends on the nature of the expected outcome: Whether it has already happened or is yet to happen.

A cohort study can be prospective or retrospective. 

Prospective cohort studies focus on outcomes that are yet to happen, whereas retrospective cohort studies use data from the past. 

For a prospective cohort study, the researcher must identify and recruit a new group of people and observe them to get new information. 

An example of a prospective cohort study is observing a group of students who eat a lot of sweets and another that doesn't. The most probable outcome is that some students will develop diabetes due to a high sugar intake.

Since the researchers already know the outcome, a retrospective cohort study’s goal is to determine the exposure by using the past as the basis for the research. 

Here, the researchers would identify a group who already have diabetes and try to figure out what actions they may have in common. The researchers may identify a risk factor of consuming large quantities of sugar in their youth.

Scientists conduct cohort studies worldwide. Cancer-focused cohort studies are some of the most common, aiming to identify and eliminate the root causes of cancer. 

Some historical US cohort studies are:

New York University Women's Health Study

At the beginning of the New York University Women's Health Study (NYUWHS), the study monitored 14,291 women aged 35–65. 

All these women hadn't been on any hormonal medications or pregnant six months before the test. The researchers screened and monitored them for breast cancer.

The Nurses' Health Study

In 1976, several US female nurses aged 30–55 were involved in this cohort study. The study aimed to evaluate the risk factors for cancer, such as oral contraceptives. 

Later in 1989, 116,671 registered female nurses aged 25–42 were involved in this study before the nutritional part of the cohort study began in 1990.

California Teachers Study (CTS)

California has a retirement system for teachers and school professionals known as the California State Teachers Retirement System (STRS). 

In 1995, the CTS began evaluating how factors such as diet, smoking, and alcohol intake impacted breast cancer risk among the teachers in the STRS.

The Black Women's Health Study (BWHS)

Since its establishment in 1995, the BWHS has been the most significant medical study for Black women. This study examines risk factors for cancer and other serious illnesses, such as: 

• Dieting

• Physical activity

• Smoking

• Oral contraceptives

The very nature of being human means scientists have a lot to study. Worldwide, people take part in countless activities. Some of these activities can lead to poor health. 

For example, people consume thousands of diverse food substances. Not everything we eat is good for us, whether that’s junk food or contaminated grains.

According to the World Health Organization (WHO), over 200 diseases come from food contaminated with bacteria and chemical substances. Cohort studies establish the relationship between such foodstuffs and people's health. 

These studies come in handy, especially when we believe a deadly disease transmits through the water or food we consume, such as cholera. 

Whenever a new disease appears out of nowhere, a cohort study will help scientists determine its root cause. This helps them develop solutions.

One of the main advantages of cohort studies is that they enable researchers to study numerous outcomes linked to single or multiple exposures. 

Other advantages of conducting a cohort study are:

Gathering data

The main reason for conducting a cohort study is to collect data on a specific exposure and the related outcomes. This data informs decision-making and improves efficiency. For example, cohort data can be crucial for developing a vaccine for a virus. 

Examining outcomes

Since a cohort study provides data for researchers, it makes analyzing the many exposures and outcomes easier. Cohort studies are crucial to research, especially for an unknown exposure.

Simplifies rare occurrences

Some rare exposures are unknown even to scientists, such as COVID-19. When these occur, a cohort study helps scientists evaluate the possible causes of the disease and learn more about what it is and how to control it.

Estimates risk

Risk estimation is a key concept researchers use to evaluate how likely a given outcome will affect an exposed or unexposed person. 

During a cohort study, researchers experience some challenges. 

For a rare, novel exposure like COVID-19, scientists needed many people to conduct a cohort study. This is a significant problem with new exposures, as a large-scale cohort study may be hard to do.

What's worse, the researchers may make an error while choosing their participants and end up with the wrong results. Prospective and retrospective cohort studies also have separate issues. 

Disadvantages of prospective cohort studies

• It may be expensive if it involves more people.

• It may take a long time to do a follow-up.

• A follow-up may be difficult.

• Prospective cohort studies are prone to losses and follow-up withdrawals.

Disadvantages of retrospective cohort studies

Retrospective cohort studies require researchers to use existing data. Even so, researchers may select the wrong people to conduct the study and record the incorrect results.

Retrospective cohort studies leave little room for researchers to choose whoever they want to do the study with since the outcomes are already there. 

This is a problem since a researcher is limited in determining the outcome regardless of the exposure. Many causes for a single outcome may complicate the research process.

A cohort study is an effective way for researchers to monitor how different elements and activities affect human health. Cohort studies take time since they examine trends and information over a long period, but they are worth the trouble.

They are essential during a health crisis, especially disease outbreaks that need immediate attention. However, they may be costly and time-consuming, especially if they involve a large group of people.