Study Design
Cohort study design
These determines or tests the relationship between several variables to suggest or establish possible causes of problems. They describe the distribution of disease in human population and investigate possible etiological factors to explain that distribution.
cohort or (follow up) studies are those in which people are identified and grouped with respect to whether they have been exposed to specific factor.
Those groups are followed up over time to determine whether the incidence of a particular disease is any greater (or less) in the exposed group than in the non-exposed group, the relative risk (incidence risk or incidence rate) is used to assess whether the exposure and disease are causally linked.
Cohort studies may be prospective or retrospective
A prospective cohort study is also called a concurrent cohort study, where the subjects have been followed up for a period and the outcomes of interest are recorded.
In a retrospective cohort study both the exposure and outcome have already occurred at the outset of the study. While this type of cohort study is less time consuming and costly than a prospective cohort study, it is more susceptible to the effects of bias. For example, the exposure may have occurred some years previously and adequate reliable data on exposure may be unavailable or incomplete. In addition, information on confounding variables may be unavailable, inadequate, or difficult to collect.
How a cohort study be conducted:
Selection of study groups:
The aim of a cohort study is to select study participants who are identical apart from their exposure status. All study participants must be free of the outcome under investigation and have the potential to develop the outcome under investigation.
Measuring exposure:
Levels of exposure (e.g., packs of cigarettes smoked per year) are measured for everyone at baseline at the beginning of the study and assessed at intervals during the period of follow-up. When several exposures are being considered simultaneously, the non-exposed group should comprise all those with none of the risk factors under investigation.
Exposure data may be obtained from several sources including medical or employment records, standardized questionnaires, interviews and by physical examination.
Measuring outcome:
Outcome measures may be obtained from various sources, including routine surveillance of cancer registry data, death certificates, medical records or directly from the participant. Note that the method used to determine outcome must be identical for both exposed and unexposed groups.
Methods of follow-up:
The follow-up of study participants in a cohort study is a major challenge. A great deal of cost and time is required to ensure follow-up of cohort members and to update measures of exposures and confounders, in addition to monitoring participants' health outcomes. The failure to collect outcome data for all members of the cohort will affect the validity of study results.
Potential sources of bias in cohort studies:
Selection bias may be introduced when the completeness of follow-up or case ascertainment differs between exposure categories. This can be minimized by ensuring that a high level of follow-up is maintained among all study groups.
Strength and weakness in cohort studies:
Strength:
can look at multiple exposures.
Exposure is measured before the onset of disease (in prospective cohort studies).
Can measure incidence and prevalence.
Weakness:
costly and time consuming.
Prone to bias due to loss to follow-up.
Prone to confounding.
Participants may move between one exposure category.
Being in the study may alter participant's behavior.
Poor choice for the study of a rare disease.
Classification of individuals (exposure or outcome status) can be affected by changes in diagnostic procedures.
By: Amjad Alharthi