Retrospective Cohort Study: Definition and Explanation
A retrospective cohort study is conducted when the investigator has formed the outcome of interest already. Here, they move back to a time where they can recognize a cohort of participants at a time period prior to the development of outcome of interest and it is at that time that they attempt to develop their exposure status. After this they are able to find out if the subject was able to develop the outcome of interest subsequently or not.
Testing the Hypothesis in Retrospective Cohort Study
If for example a researcher wants to test the hypothesis that if one works with the chemicals used in the manufacturing of tires, it raises the risk of death. As it is a relatively rare exposure, it is advised to make use of a special exposure cohort in this case like the employees of a huge tire manufacturing industry. The employees who were working in reality with the chemicals which are involved in the process of manufacturing tires they would be the exposed group. On the other hand, the management as well as the clerical staff will be a part of the unexposed group. But instead of following such subjects for a number of years, it will be more accurate to make use of the health of employees along with their employment records for the previous two or more decades which will serve as important data sources.
The Essence of a Retrospective Cohort Study
The essence of a retrospective cohort study states that the researcher has to move back into a time where they can determine the study cohort at a particular time prior to the occurrence of the outcome of interest which is death in the above example. After this the researcher categorizes them as unexposed or exposed on the basis of their employment records. Different sources can be used to find out the outcome status like death. Examples of this can be the next of kin, use of health records, National Death Index and a lot more.
Purpose of Retrospective Cohort Study
A retrospective cohort study like the one which is mentioned above is very beneficial in terms of observing unusual and rare exposure however there are several issues here. There are times when there is a lack of clarity in the exposure status when it is important to move back into time to utilize all the data that is present, particularly due to the fact that the data that is used was not operated to answer the health question.