Lecture 19: Cohort Studies

Question 1

What is a cohort study?

Answer 1

A study where individuals are defined on the bases of presence or absence of exposure to a suspected risk factor

Question 2

What type of study is a cohort study?

Answer 2

Analytic and observational

Question 3

What is involved in an observational study?

Answer 3

where you observe people’s exposures and what happens to them

Question 4

What are the 6 steps to carrying out a cohort study?

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)
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 measures of occurrence can we measure using cohort studies?

Answer 5

Incidence proportion and incidence rate

Question 6

What measures of association?

Answer 6

• relative risk (using either incidence rate or incidence proportion)
• risk difference (using either incidence rate or incidence proportion)

Question 7

What are some strengths of cohort studies?

Answer 7

• can determine temporal sequence between the exposure and the outcome.
• can examine multiple outcomes from an exposure
• can calculate incidence so we can also evaluate relative risk difference
• good for studying rare exposures

Question 8

What are some weaknesses of cohort studies?

Answer 8

• loss to follow up as it occurs over a long period of time which can lead to bias.
• potential for misclassification of exposures and outcomes
• not good for classifying rare outcomes.
• can be time consuming and expensive

Question 9

What do we need to consider when we are recruiting our sample population? Give an example to explain why

Answer 9

Ideally, we want a random selection independent of the exposure status but sometimes the selection is based on the exposure status and therefore we must carefully consider the appropriate comparison group which can lead to the healthy worker effect. If we use workers as a comparison group, they may be healthier than the rest of the population.
We also need to be careful that the sample population does not already have the outcome.

Question 10

What do we need to consider when we are assessing the exposure to identify which group participants belong in?

Answer 10

have the participants been correctly classified

Question 11

What three things do we need to consider when we follow up over time?

Answer 11

1. have the participants changed their exposure status?
2. has everyone been followed up over the entire study?
3. how long do participants need to be followed up?

Question 12

What do we need to consider when we are observing whether or not participants develop the outcome?

Answer 12

has the outcome status been correctly classified

Question 13

What’s the difference between a prospective cohort study and a historical cohort study?

Answer 13

• prospective is when you start of without the outcome and we see if you develop it
• historical is when we know that you have the outcome and we can see when you developed it

Question 14

What are the advantages of a historical cohort study?

Answer 14

• less time consuming compared with prospective cohort studies
• good for outcomes that take a long time to develop
• less expensive

Question 15

What are the disadvantages of a historical cohort study?

Answer 15

• it uses existing data which may be poor and unreliable
• we may not know about all relevant factors
• there can be selection bias

Question 16

What is the difference between case control studies and historical cohort studies?

Answer 16

In a case control study, the groups are split by outcome but in a historical study, they are split by exposure

Question 17

Cohort studies are what type of studies?

Answer 17

observational and analytic