Cohort studies

Question 1

Where in the heirachy of evidence do cohort studies lie?

Answer 1

Second

Question 2

What is a cohort?

Answer 2

sample from population without the outcome of interest (exposure is measured at baseline and they are followed up to see if those exposed to the exposure had more/less of a risk to develop outcome -> risk ratios)

Question 3

What are the sources of both exposure and outcome data?

Answer 3

Health records

Interviews

Quiestionnaires

Physical examinations

Question 4

What are sources of exposure data only?

Answer 4

Employment records

Question 5

What are sources of outcome data only?

Answer 5

Death certificates

Question 6

What are the two types of cohort study?

Answer 6

Prospective (those exposed/not followed up to observe incidence of outcome)

Historical (data on exposure from pre-existing historical records -> follow subjects with records availiable to see if developed outcome of interest at any time up to the present)

Question 7

Which takes less time to undertake… Historical or prospective cohort studies?

Answer 7

Historical -> do not need to wait for the outcome to occur

Question 8

How do the two types of cohort studies compare?

Answer 8

• Historical = quicker

BUT

• Historical records may not be availiable = not possible
• Exposure data not collected for research purpose = possibly poor quality

Question 9

What is risk?

Answer 9

Risk = no. new cases of outcome in time period / no. in population without outcome at outset

Calculated seperately for both exposed and unexposed group

= porbability between 0 & 1

Question 10

What is risk ratio?

Answer 10

risk ratio = risk in exposed group/risk in unexposed group

Question 11

How do you interpret risk ratio?

Answer 11

RR = 1 -> no difference between exposure groups

RR = 0.5 -> exposure halves risk

RR = 0.75 -> exposure reduces risk by 25%

RR < 1 exposure reduces the risk

RR = 2 -> exposure doubles the risk

RR = 10 -> exposure increases the risk ten fold

RR > 1 = exposure increases the risk

Question 12

What does risk ratio indicate?

Answer 12

Strength of association between exposure and outcome

BUT

associations may not be causal

Question 13

Which two things should be used to interpret risk ratio values calculated from cohort studies?

Answer 13

P values (measure strength of evidence against null hypothesis)

CI intervals (assess orecision of estimated RR)

Question 14

How do you test for a trend?

Answer 14

Calculate RR for each category compared to baseline

RR in baseline group = 1

If there is an association/trend increase in exposure = increase in outcome

Question 15

What are the two different types of trend?

Answer 15

Linear trend (increase risk in outcome assiciated with increase or decrease in exposure)

Threshold effect (risk of outcome increases only for those subjects whose exposure is above or below a certain level)

** INSERT PIC ***

Question 16

What is risk difference?

Answer 16

Risk difference = risk of outcome occurence in exposed - risk of outcome occurence in unexposed

Question 17

Which values can be calculated form risk differences?

Answer 17

CI intervals

P values

Question 18

What is the difference between risk ratios and risk differences?

Answer 18

Risk ratio measures strength of association between exposure and outcome (relative measure = no units)

Risk difference measures impact of exposure on outcome (has units) -> depends on length of study = cannot compare different studies

Question 19

Which is more preferrable… risk ration or risk differences?

Answer 19

Risk ratio

because it is NOT depenent on length of study

Question 20

In a prospective cohort study what can measurement bias affect?

Answer 20

• Unlikely to affect exposure as outcome is unknown at the time of measurement
• May affect outcome as exposure of group will be known

Question 21

What are the strengths of (prospective) cohort studies (5)?

Answer 21

• Exposure preceeds outcome
• Outcome incidence measured directly
• Can test hypotheses about >1 outcome
• Selection bias unlikely (selected as a single gorup then classified accoridng to exposure)
• Measurement bias for exposure unlikely

Question 22

What are the weaknesses of a (prospective) cohort study (6)?

Answer 22

• Time consuming
• Expensive
• Rare outcomes require very large sample sizes
• Changes over time can affect exposure and outcome classification
• Measurement bias for outcome
• Loss to follow-up bias (rate of loss of subjects may be related to outcome)