The cohort studies and the challenges of follow-ups

A cohort study is a type of observational study that tracks a group of people with different exposures and characteristics during a time and measures the outcomes of interests at one or more time points. These outcomes are different from one study to another. For example, in public health studies, these can be health level, disease, or mortality rate. In a cohort study, a researcher compares the outcomes between subgroups of that study.

These studies are specifically helpful for investigating the effects of exposure or risk factors on people’s health when other types of studies (like randomized controlled experiments are not possible or have ethical issues). However, it also faces specific challenges, specifically regarding follow-up. As a researcher, before starting a cohort study, you should be completely aware of possible challenges you may face in such a journey. So, in this article, we introduce possible follow-up challenges you may face in a cohort study.

What does follow-up mean, and how is it important?

Follow-up in a cohort study means monitoring enrolled people during the time and collecting their exposure status data, outcome status data, and any other variable determined to collect before starting the survey. Although it is vital to ensure the validity and reliability of the results, it could be a complicated process since it is cost- and time-consuming depending on the design and study duration.

Unlike some cohort studies that can easily use subjects and end up in a couple of hours, many cohort studies need a large sample size and prolonged follow-up time since their outcomes may have low incidence or long latency periods. For example, the occurrence of cardiovascular diseases, Alzheimer’s and dementia, hearing loss, etc., is low. On the other hand, the patients should be follow-up during the study, and their changes or possible treatments should be recorded; all these make such a study complex and expensive.

It is noteworthy that designing a cohort study needs a team of multidisciplinary experts depending on the field such study focuses on. For example, for a cohort study regarding the prevalence of dementia in a specific county, public health nurses, public officials, physicians, and possible agents of community residents are needed. So, a well-organized team must gather to ensure practical cooperation among them. So the leaders of such research should be aware of the administration of this complex system.

Types of cohort studies

There are two main types of cohort studies: prospective and retrospective.


In this type of study, the study’s baseline is determined at the beginning of the study, and the group is followed into the future.


The baseline exposure of a retrospective cohort study, a so-called historical cohort study, is assessed at a time point in the past through historical records. For example, blood pressure records of heart disease patients of a specific hospital may provide exposure and outcome information up to now.

Main challenges of follow-up in cohort studies:

As mentioned earlier, cohort studies face many challenges, from administrating a multidisciplinary team to follow-up participants. The main challenges of the first one are related to managing the research leader’s capabilities, while the latter’s challenges are entirely different. Some of the significant follow-up challenges participants in a cohort study are as follows:

Loss to follow-up

This problem occurs when enrolled people drop out of the study or researchers cant connect or trace one or more participants to collect data. This issue may produce bias and decrease the study’s statistical power in cases of participants with different specifications or results compared to those who remained in the study. Researchers can avoid this problem by using different contact methods, implementing incentives, and using remainders and multiple tracking systems.

Competing events

These events hinder the occurrence or observation of the outcome of the study. For example, in a cohort study on blood pressure, dementia, or any health cohort study, death would be a competing event. Such events influence estimating and interpreting the resultant incidence and survival probability. You can avoid this challenge using competing risk analysis or case-specific hazard analysis.

Changes in exposure status

This challenge happens when enrolled people alter their exposure level or category during the time of the study. For instance, In a cohort study on heart disease in which smoking habit is considered variable, smokers who quit smoking or none smokers who start smoking in the follow-up period are all examples of changing exposure statutes. Such changes may influence the categorization and comparison of exposure groups and interrupt the relation between exposure and outcome. A researcher can overcome such challenges by using methods like time-varying covariates analysis or multiple exposure measurements.

Changes in outcome definition

This challenge happens when the criteria or methods for recognizing or measuring the outcome alter over time. More precisely, when a new diagnostic test or screening plan is introduced in the follow-up period, this may influence the validity and computability of the result data and produce bias or misclassification. You can handle such challenges using standardized and consistent definitions as well as that method for result assessments.

Alteration confounding factors

This issue occurs when another variable influences exposure and result change during the follow-up period. Socioeconomic status, lifestyle factors, and co- or multi-morbidities are confounding factors that may alter during the study period. Altering such factors may influence the adjustment and control of confounding and interrupt the relation between exposure and outcome. To avoid such problem, you can use methods like multivariable regression analysis or stratified analysis with repeated measurements of confounding factors,

Ethical issues

This issue happens when possible hurts or benefits are related to the exposure or result, which may impact participants’ health or autonomy. For instance, participants may face adverse effects from exposure or benefit from preventing or therapeutic interventions of the outcome. This kind of follow-up challenges may influence the satisfaction and participation of participants as well as the responsibility and liability of researchers. However, you can overcome such issues by following ethical principles and guidelines, getting informed consent, ensuring participants’ confidentiality and privacy, providing feedback and support, and monitoring and reporting adverse events.


The fundamental aspect of a cohort study is follow-up. It is a time and cost-consuming process which requires an expert and multidisciplinary team. In addition, follow-up participants during the study period face several challenges that may affect the study’s quality and credibility. These challenges need to be predicted and addressed by researchers using appropriate methods

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