In the clinical trials arena, the observational studies are among the important categories of study designs. This is because; in order to address some of the investigative questions, the randomized controlled trials will not always be indicated. In these cases, the observational studies will be the best way to address those investigative questions.
Cohort and case-control studies are the two major types of observational studies which is helpful in evaluating the association between various diseases and exposures. (1) In this article, we will learn about cohort and case-control studies in detail:
A cohort study can be otherwise called as a form of longitudinal study. In medicine, the cohort study can be understood as an analysis of risk factors. Any cohort study will follow a group of trial participants and can be compared to the cross-sectional studies.
The term 'cohort' which is derived from Latin can be defined as a group of people who are sharing a common characteristic and/or experience. Most often, the cohort studies are conducted for obtaining evidence and are undertaken where the case study approach will not be feasible. (1), (2), (3)
Any well-designed cohort study will be able to provide you with more powerful results. As such, in the cohort studies, an outcome will be first identified by the exposure and/or event of interest which will be followed until the disease or outcome occurs. Yes, the cohort will be identified before the disease under investigation occurs. And, therefore a group of people who do not have the disease will be followed in a cohort study. (1), (2)
Steps Followed in a Cohort Study:
Following are the different steps that are being followed in a cohort study:
- Identification of the study subjects who constitutes the cohort population
- Obtaining the baseline data on the exposure
- Measuring the exposure in the beginning
- Selection of sub-classification in the cohort - The unexposed control cohort will be the control group
- Measuring the outcomes with the help of records, interviews, and examinations
- Data analysis - where the outcomes are being assessed and compared (4)
The cohort studies can either be prospective or retrospective:
What is a prospective cohort study?
In a prospective cohort study, the trial investigator would begin the study with the identification of a cohort population along with the exposure status. Then, the study population will be followed for the development of disease. More usually, the cohort studies will take a long time to get completed.
What is a retrospective cohort study?
In this type of cohort study, the investigator will make use of the existing data that is collected in the past in order to identify the cohort population as well as the exposure status. With these data, the investigator will determine the current status of the disease. The retrospective cohort studies will usually take a short time to be completed. (5)
Strengths of Cohort Study:
- Multiple exposures can be looked
- This is good for the measurements of rare exposures
- Incidence as well as the prevalence can be measured
- For one exposure, multiple outcomes can be measured (9)
A case-control study is another type of observational study that is being used more widely these days. These types of studies were originally developed in the field of 'epidemiology'. These studies are designed specifically for determining if an exposure is linked with an outcome. In a theoretical point of view, a case-control study can be described in a simple way:
- Identification of the cases which will be the group of people known to have the outcome
- Identification of the controls which will be the group of people known to be free of outcome
- Looking back in time for:
o Learning which subjects in each group had the exposure
o Comparing the frequency of exposure in case group to control group
By the above definition of case-control study, it can be understood that the case-control studies are always retrospective since it begins with an outcome which then traces back for investigating the exposures. And, when the trial subjects are enrolled into their respective groups, the outcomes of all the subjects will be known to the investigator. (6), (7)
The case-control studies would identify the subjects by the outcome status at the start of the investigation. In these studies, the outcomes of interest may be whether the subjects:
- Have undergone a specific type of surgery
- Have experienced a complication
- Are diagnosed with a disease
Once the status of outcome has been identified and after the subjects are categorized as case and control groups, the data about the exposure to the risk factor will be collected retrospectively. This may be in any of the following forms:
- Abstraction from records
Thus, a case-control study is well-suited for investigating the rare outcomes and the outcomes with long latency periods. And hence, these studies are quick and extensively inexpensive to implement when compared with the cohort studies. (1)
Selection of cases in the case-control studies:
This will be the starting point for any case-control study and would require a suitable case definition. Also, care needs to be taken such that bias is not arising from the way the cases are selected. Hence, the definition of a case should be very specific. (7), (8)
Selection of controls in the case-control studies:
More usually, it will not be difficult to obtain the suitable sources of cases. But, the selection of controls will be more problematic. Controls that are selected should be the people who are similar in several ways to the cases. Also, the selected control group should be at similar risk of developing the outcome of interest. Furthermore, the exposures of the controls must be measurable with similar accuracy as that of the cases.
Strengths of case-control study:
- Cost-effective when compared to other analytical studies
- No longer follow-up periods
- Good for studying the diseases with longer latency periods
- Efficient for studying the rare diseases (10)