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Question 1

Analytic epidemiology:

Answer 1

Analytic epidemiology: exposures and outcomes

Question 2

Observational:

Answer 2

observe people’s exposures and what happens to them

Question 3

What is a cohort study?

Answer 3

“ …individuals are defined on the basis of presence or absence
of exposure to a suspected risk factor”

Question 4

Cohort studies step by step

Answer 4

1. Identify a source population
2. Recruit your sample population (**Sample population must not already have the outcome of interest)
3. Assess exposure to identify which group participants belong in (exposed or not exposed i.e. comparison)
4. Follow up over time
5. Observe whether or not participants develop the outcome
6. Calculate measures of occurrence and measures of association

Question 5

What can we measure using cohort studies?

Answer 5

Incidence proportion (IP)
Incidence rate (IR)
(Measures of occurrence)

Relative risk
Risk difference (attributable risk)
(Measures of association)

Question 6

Some examples of cohort studies

Answer 6

Question 7

What does PECOT stand for?

Answer 7

Population
Exposure
Comparison
Outcome
Time

Question 8

1. Identify a source population
2. Recruit your sample population

What might we need to concider?

Answer 8

Ideal: random selection independent of exposure status

Sometimes: selection based on exposure status, (must consider appropriate comparison group)

Healthy worker effect

Can you be sure that the sample population does not already have the outcome?

Question 9

3. Assess exposure to identify which group participants belong
   in (exposed or not exposed i.e. comparison)

What might we need to consider carefully?

Answer 9

Have participants been correctly classified?

Question 10

4. Follow up over time

What might we need to consider carefully?

Answer 10

Ø Have participants changed exposure status over time?
Ø Has everyone been followed up over the entire study?
Ø How long do participants need to be followed up?

Question 11

5. Observe whether or not participants develop the outcome

What might we need to consider carefully?

Answer 11

Has the outcome status been correctly classified?

Question 12

Strengths of cohort studies

Answer 12

Determine temporal sequence between exposure and outcome

Can examine multiple outcomes from an exposure

Can calculate incidence (and therefore relative risk and risk difference)

Good for studying rare exposures

Question 13

Limitations of cohort studies

Answer 13

Loss to follow up (Can lead to bias if related to the exposure and the outcome)
• Potential for misclassification of exposures/outcomes
• Generally not good for studying rare outcomes
• Time consuming
• Can be expensive

Question 14

Two types of cohort studies

Answer 14

1 . Prospective cohort studies
2. Historical cohort studies (Sometimes called retrospective cohort studies)

Question 15

The difference in starting point in prospective vs historical cohort studios

Answer 15

Question 16

What is the difference between a prospective and retrospective cohort study?

Answer 16

The starting point

Question 17

Features of a historical cohort study

Answer 17

• Use existing data
• Reconstruct follow-up period in the past

Question 18

Pros and cons of historical cohort studies

Answer 18

PROS
• Less time consuming compared with prospective cohort studies
• Good for outcomes that take a long time to develop
• Less expensive

CONS
• Use existing data (collected for other reasons) – quality?
• May not know about all relevant factors
• Bias?

Question 19

the difference between historical cohort studies, case-control studies and cross-sectional studies

Answer 19

historical cohort studies focus on a group with a shared past exposure and follow them into the future, case-control studies start with individuals who already have an outcome and compare them to controls to identify risk factors, and cross-sectional studies provide a snapshot of a population’s characteristics at a single point in time.