Study design: cohort and case control studies

If the odds of being exposed in the cases is 5/4 and the odds of being exposed in the controls is 4/5, what is the odds ratio?

Odds ratio = (5/4)/(4/5) = 25/16

What is the calculation for risk ratio?

RR = a/(a+b)/c/(c+d).

Can confounding occur in these 2 types of study?

Yes, in both types.

List 5 features of case-control studies.

1. Observational analytical studies. 2. They have a retrospective design. 3. Controls are selected to represent a source population of cases. 4. Exposure determined post diagnosis. 5. Odds ratio is a measure of relative risk.

State the difference between prospective and retrospective cohort studies.

A prospective cohort study ascertains disease during follow up, whereas a retrospective cohort study looks at events that have already happened.

Define a "cohort".

A cohort is a group of people who have something in common. They can represent the disease-free population from which cases with the disease eventually arise.

What can the retrospective design of case-control studies lead to?

Recall bias.

State 2 features of the comparison (control) group in a case-control study.

1. They should come from the population of individuals who would have been identified and included as cases had they also developed the disease. 2. They should be representative of that population.

List 5 strengths of a cohort study.

1. Able to look at multiple outcomes. 2. Able to follow through natural history of disease. 3. Good design to look at risks relating to rare exposures. 4. Incidence can be directly calculated. 5. Can minimise bias in estimating exposure if prospective.

Describe the procedure of case-control studies.

1. From source population, select cases and controls. 2. Subdivide cases and controls into whether or not the subject has been exposed. Obtain information on past exposures and other factors. 3. Compare proportions of people exposed in cases and controls, and calculate an odds ratio for cases/controls exposure.

List 5 strengths of case-control studies.

1. Good for rare disease. 2. Quick. 3. Cost efficient. 4. Can investigate many exposures simultaneously. 5. Good at examining diseases with long latency periods.

List 4 weaknesses of cohort studies.

1. Inefficient for studying rare diseases. 2. Expensive and time consuming (if prospective). 3. Loss to follow up may introduce bias. 4. Healthy worker effect may cause bias in occupational cohorts.

List 6 features of a cohort study.

1. Observational analytical study. 2. A group of people (cohort) followed over time. 3. Prospective or retrospective design. 4. Exposures are measured prior to disease (prospective). 5. Retrospective cohort studies use previously recorded information on exposures. 6. Can directly measure incidence of disease in exposed and unexposed people, which can be used to calculate rate or risk ratios.

Describe the procedure involved in conducting a cohort study.

1. Select individuals without the disease of interest into exposed and unexposed groups. 2. Follow the cohorts over time and determine how many people got disease over a certain time. 3. Compare the risks of disease in the different cohorts using relative risk calculation.

List 5 weaknesses of case-control studies.

1. Selection bias during selection of controls. 2. Recall bias during investigations of exposures. 3. Uncertainty of exposure-disease time (temporal) relationship. 4. Poor for rare exposures. 5. Cannot calculate incidence rates directly.

What does it mean if RR<1?

The exposure is associated with a decreased risk of the disease.

If odds ratio is greater than one, what can be inferred?

The exposure is associated with an increased risk.

What is the most difficult part of a case-control study?

The selection of an appropriate comparison group.