Difference between Case control study and Retrospective cohort study

Case control study inspects individuals by outcome/disease status. But, the retrospective cohort study inspects individuals by their exposure status.

Case control study

Case–control study is a type of observational study where individuals with outcome of interest (cases) and individuals without outcome of interest (controls) are first identified. Then these groups are studied retrospectively to compare the frequency of the exposure to a risk factor in order to estimate the relationship between the risk factor and the subsequent outcome. Case control study is also known as "retrospective study" and "case-referent study".

Example of Case control study

The researcher selects few cancer patients and non- cancer patients and studies the histories of both groups to compare the frequency of exposure to risk factors i.e., smoking for each group in order to estimate the relationship between the risk factor (smoking) and the disease (lung cancer).

Here, the researcher begins his/her study with the outcomes (cases and controls) and tries to determine the exposure details in both the groups.

Retrospective cohort study

A retrospective cohort study is a longitudinal cohort study where a cohort group of individuals who share a common characteristic or exposure is identified and studied with the help of past records to determine the effect of the exposure on the development of current outcome. Retrospective cohort study is also known as “historic cohort study”.

Example of Retrospective cohort study

The researcher identifies a cohort of individuals who share a common exposure factor such as habit of smoking. He then examines the past records of these individuals (smokers and non-smokers) to understand the effect of the smoking on the development of current outcome i.e., lung cancer. He classifies the sample into two groups i.e., exposed to risk factors (smoking) and not exposed to risk factors (no smoking) to study the relationship.

Here, the researcher begins his study with the past records of exposure of the individuals to find out the possible link between the exposure to risk factor and the present outcome.

Retrospective cohort study starts after the target population have already developed the outcomes of interest. The researcher travels in past and tries to read the past records of the selected cohort at a point in time before they have developed the outcomes of interest. Then the researcher examines the exposure details (the form and time of exposure to a factor, the latent period, the time of any subsequent occurrence of the outcome, etc) of the cohort at that point in time.

There is no follow-up process with the sample however the starting point for retrospective cohort study will be the same as other cohort studies i.e., researcher starts studies when the outcome of interest has not developed.

Thus, retrospective cohort study works when all the details such as outcome, exposure, etc are available. The researcher just jumps into the past and reads and analyses the association of exposure and outcome.

Case control study Vs. Retrospective study

Case-Control Study uses subjects who have a particular outcome (cases) and do not have particular outcome (controls) and looks back (retrospectively) to determine what the exposure was. Whereas, retrospective cohort study uses historical data to explain the exposure level at some baseline in the past and then explains the subsequent outcome/disease in the present.

Case-Control Study starts with the study of the outcome/disease and it checks if it is associated with the exposure of risk factors. This type of study is typically used for rare diseases where comparison of cases (with rare disease) is done with a control group (without disease). Hence, case control study can use odd ratio. On the other hand, retrospective cohort study starts with the study of the exposure to a risk factor and it checks if it has played a role in development of current outcome. It describes a degree of risk (probability) of disease occurrence due to exposure compared to non-exposure i.e. the relative risk. Hence, relative risk and attributable risk can be used in retrospective cohort study.