Study Designs - Lecture Nineteen

Question 1

Cohort Studies step one

Answer 1

Identify a source population

Question 2

Cohort Studies step two

Answer 2

Recruit your sample population

Question 3

Cohort Studies step three

Answer 3

Assess exposure to identify which group participants belong to

Question 4

Cohort Studies step four

Answer 4

Follow up overtime

Question 5

Cohort Studies step five

Answer 5

Observe whether or not participants develop the outcome

Question 6

Cohort Studies step six

Answer 6

Calculate measures of occurrence and measures of association

Question 7

What can we measure using cohort studies?

Answer 7

Measures of occurrence
- Incidence proportion 
- Incidence rate
Measures of association
- Relative risk
- Risk difference

Question 8

Incidence proportion

Answer 8

Number of people who develop the disease in a specific period / Number of people at risk of developing the disease at the start of the period

Question 9

Incidence rate

Answer 9

Number of people who develop the disease in a specific period / Number of person-years at risk of developing the disease

Question 10

Relative risk

Answer 10

How many times as likely it is that the exposed group will develop the disease compared to the comparison group
Rate ratio: IRe/IRc
Risk ratio: IPe/IPc

Question 11

Risk difference

Answer 11

How many extra/fewer cases of the outcome there were in the exposed group compared to the comparison group
IRe-IRc
IPe-IPc

Question 12

What might we need to consider carefully - Step One

Answer 12

1. Identify a source population
   Ideal: random selection independent of exposure status
   Sometimes: Selection based on exposure status (might consider appropriate comparison group)

Question 13

What might we need to consider carefully - Step Two

Answer 13

2. Recruit your sample population

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

Question 14

What might we need to consider carefully - Step Three

Answer 14

3. Assess exposure to identify which group participants belong in (exposed or not exposed i.e. comparison)
   Have participants been correctly classified?

Question 15

What might we need to consider carefully - Step Four

Answer 15

4. Follow up over time
   Have participants changed exposure status over time?
   Has everyone been followed up over the entire study? Otherwise lost to follow up
   How long do patients need to be followed up?

Question 16

What might we need to consider carefully - Step Five

Answer 16

5. Observe whether or not participants develop outcome

Has the outcome status been correctly classified?

Question 17

Strengths of cohort studies

Answer 17

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 18

Limitations of cohort studies

Answer 18

Loss to follow up
Potential for misclassification of exposures/outcomes
Generally not good for studying rare outcomes
Time consuming
Can be expensive

Question 19

What could be done instead of cohort studies?

Answer 19

Case-control studies

Historical cohort studies

Question 20

Types of cohort studies

Answer 20

Prospective cohort studies

Historical cohort studies

Question 21

Prospective cohort studies

Answer 21

The research starts with classifying the exposure

Question 22

Historical cohort studies

Answer 22

The exposure and outcome have already happened

Question 23

Historical cohort studies strengths

Answer 23

Use existing data
Reconstruct follow-up period in the past
Less time consuming compared with prospective cohort studies
Less expensive
Good for outcomes the take a long time to develop

Question 24

Historical cohort studies limitations

Answer 24

The quality of existing data
May not know all relevant factors
Selection bias