Certain advantages and disadvantages exist for each of the various study designs. Compare and contrast the advantages and disadvantages of case control and cohort study designs (15 marks).
Comment: In this case, we give advantages and disadvantages of cohort and case control study designs and offer a summary table to bring out their strengths and shortcomings.
Cohort studies
Advantages
1. Allow complete information on the subject’s exposure, including quality control of data, and experience thereafter.
2. Provide a clear temporal sequence of exposure and disease.
3. Give an opportunity to study multiple outcomes related to a specific exposure.
4. Permit calculation of incidence rates (absolute risk) as well as relative risk.
5. Enable the study of relatively rare exposures.
Disadvantages
1. Not suited for the study of rare diseases because a large number of subjects is required.
2. Not suited when the time between exposure and disease manifestation is very long, although this can be overcome in historical cohort studies.
3. Exposure patterns, for example the composition of oral contraceptives, may change during the course of the study and make the results irrelevant.
4. Maintaining high rates of follow-up can be difficult.
5. Expensive to carry out because a large number of subjects is usually required.
6. Baseline data may be sparse because the large number of subjects does not allow for long interviews.
Case-control studies
Advantages
1. Permit the study of rare diseases.
2. Permit the study of diseases with long latency between exposure and manifestation.
3. Can be launched and conducted over relatively short time periods.
4. Relatively inexpensive as compared to cohort studies.
5. Can study multiple potential causes of disease.
Disadvantages
1. Information on exposure and past history is primarily based on interview and may be subject to recall bias.
2. Validation of information on exposure is difficult, or incomplete, or even impossible.
3. By definition, concerned with one disease only.
4. Cannot usually provide information on incidence rates of disease.
5. Generally incomplete control of extraneous variables.
6. Choice of appropriate control group may be difficult.
Comparison of strengths and weaknesses of cohort and case control studies
Characteristics Cohort studies Case control studies
Suited for rare diseases No Yes since starting with a set of cases
Suited for rare exposures Yes since starting with exposure status No
Allows for studying several exposures Difficult but examples exists
(Framingham study) Yes
Allows for studying several outcomes Yes No
Disease status easy to ascertain Sometimes difficult Easier since starting point of the study
Exposure status easier to ascertain Yes since starting point of the study.
Except for retrospective cohorts Sometimes difficult.
Information biases.
Allows computation of risk and rates Yes No
Allows computation of effect Computation of risk ratio and rate ratio Estimation of risk ratio, rate ratio from odds ratio
Allows studying natural history of disease Yes
Easier to show that cause precedes effect. More difficult
Temporality between cause and effect difficult to establish
Based on existing data sources Difficult Yes but access to information sometimes difficult
Easiness to find a reference group Usually not difficult to identify an unexposed population No
Major potential biases when selecting a control group
Sample size Large Small
Cost Elevated except if retrospective cohorts Smaller
Time required Long, sometimes very long except if retrospective cohorts Shorter
Follow up Difficult, loss to follow up No follow up
Logistics Heavy
Many staff, large data sets
Long duration Easier
Concept Easy to understand Difficult to understand particularly if case cohort or density case control study
Ethical issues Major if studying risk factors.
Interruption of study if exposure shown to be harmful.
Need for intermediate analysis. None since outcome already happened.