Session 5

Question 0

What is a cohort?

Answer 0

• A defined group of people with known personal characteristics and exposure

Question 1

What is a cohort study?

Answer 1

• A study that starts with disease-free individuals divided into cohorts based on exposure to a pre-defined cause, that are followed up during a study period to see whether they develop an outcome.
• The risk between the exposed group and unexposed group is then compared.

Question 2

How is relative risk calculated between the exposed and unexposed groups?

Answer 2

• Internal comparison: Incidence rate ratio

- External comparison: Standardised mortality ratio

Question 3

What is an internal comparison?

Answer 3

• Sub-divide the exposure group

- Comparisons made within the cohort

Question 4

What is an external comparison?

Answer 4

• Using the external reference population as a comparison

Question 5

How are internal comparisons carried out?

Answer 5

• Work out the incidence rate for both the exposed and unexposed groups
• Calculate the incidence rate ratio
• Calculate the error factor= exp(2x✔️(1/exposed + 1/unexposed))
• Calculate the upper and lower limits of the 95% confidence interval:
  (IRR/ef) to (IRRxef)
• If the null hypothesis value (ie 1) is outside the 95% confidence interval, the p value < 0.05 and the rates of the exposed and unexposed are significantly different

Question 6

How are external comparisons carried out?

Answer 6

• Work out the SMR= (observed/expected) x100
  Observed = from cohort study
  Expected = from reference population
• Calculate the error factor= exp(2x✔️1/Observed)
• Calculate the upper and lower limits of the 95% confidence interval:
  (SMR/ef) to (SMRxef)
• If the null hypothesis value (ie 1) is outside the 95% confidence interval, the p value < 0.05 and the rates of the exposed and unexposed are significantly different

Question 7

What are some disadvantages of internal comparisons?

Answer 7

• Sub-cohorts may be radically different sizes so large studies are needed
• Sub-cohorts may not be comparable due to confounding factors

Question 8

What are some disadvantages of external comparisons?

Answer 8

• Large cohorts need to be studied over a long period of time, so the cohort members age and the age-specific mortality rates of the reference population changes
• More complicated to calculate the expected number of events (a Lexis diagram is used)
• Not enough detailed data on the reference population for comparison
• Health worker effect- study cohort and reference population are not comparable

Question 9

What are the 2 types of cohort study?

Answer 9

• Prospective/concurrent

- Retrospective/historical

Question 10

What reduces the error factor?

Answer 10

• Increasing the number of events

Question 11

Is the error factor larger for internal or external comparisons?

Answer 11

• Larger for internal comparisons
• There is uncertainty in both the incidence rates of the exposed and unexposed groups in internal comparisons, but only the observed value in the SMR is uncertain in external comparisons as there is less random variation

Question 12

Can the error factor be reduced in internal comparisons?

Answer 12

• Yes, BOTH the groups need to be increased in size

Question 13

What is an advantage of internal comparisons?

Answer 13

• More detailed information can be collected, which allows the dose-response effect to be studied