A retrospective cohort study, also called a historic cohort study,
generally means to take a look back at events that already have taken place.
For example, the term is used in medicine, describing a look back at a
patient's medical history or lifestyle. Retrospective cohort studies have
existed for approximately as long as prospective cohort studies.
Design It is a medical research study in which
the medical records of groups of individuals who are alike in many ways
but differ by a certain characteristic are compared for a particular outcome.
In retrospective cohort studies, a risk ratio or odds ratio gives an assessment
of relative risk. In the case of a retrospective cohort
study, the investigator collects data from past records and does not follow
patients up as is the case with a prospective study. However, the starting
point of this study is the same as for all cohort studies. The first objective
is still to establish two groups - exposed versus non-exposed; and these
groups are followed up in the ensuing time period.
In a nutshell, in Retrospective Cohort Study, all the events - exposure, latent
period, and subsequent outcome have already occurred in the past. We merely
collect the data now, and establish the risk of developing a disease if exposed
to a particular risk factor. On the other hand, Prospective Cohort Study is
conducted by starting with two groups at the current point, and following up in
future for occurrence of disease, if any.
It is important to understand that the methodology of prospective and
retrospective cohort studies is fundamentally the same, but the
retrospective study is performed post-hoc, as the cohort is followed
retrospectively. The time to complete a retrospective study is only as long as
it takes to collect and interpret the data. Retrospective studies examine
possible risk and protection variables in relation to a result that is already
established at the start of the study. Caution needs to be exercised, in
particular, with retrospective cohort studies because errors due to
confounding and bias are more common in retrospective studies than in
prospective studies. Advantages
Retrospective cohort studies have the following distinct advantages when
compared with prospective cohort studies: they are conducted on a smaller
scale, they typically require less time to complete, they are better for
analysing multiple outcomes, and, in a medical context, they can potentially
address rare diseases, which would necessitate extremely large cohorts in
prospective studies. In such a study, diseased people have already been
identified so retrospective studies are especially helpful in addressing
diseases of low incidence. The fact that retrospective studies are generally less
expensive than prospective studies may be another key benefit. These studies
tend to be less expensive in part because outcome and exposure have
already occurred, and the resources are directed at mainly collection of data.
Additionally, it has essentially all the benefits of a Cohort Study
Disadvantages Retrospective studies have disadvantages
vis-a-vis prospective studies. Among the disadvantages are that some key
statistics cannot be measured, and significant biases may affect the
selection of controls. Additionally, major biases with retrospective cohort
studies can impact the recall of former exposure to risk variables. Among the
biases which can negatively impact the veracity of this type of study are
selection bias and misclassification or information bias as a result of the
retrospective aspect. With retrospective studies, the temporal relationship is
frequently difficult to assess. Also, those who conduct retrospective studies
cannot control exposure or outcome assessment but instead need to rely on
others for accurate recordkeeping. That is particularly problematic because it
can be very difficult to make accurate comparisons between the exposed and the
nonexposed. Retrospective studies also may need very large sample sizes for
rare outcomes. References
This article incorporates public domain material from the U.S. National Cancer
Institute document "Dictionary of Cancer Terms".