Cohort studies

Question 1

Outline cohort study

Answer 1

Identify outcome free individuals
Identify exposure
Follow into future to see outcomes (exposed vs unexposed outcomes)

Question 2

Different types of cohort study

Answer 2

Concurrent/prospective = find people and data collect in present/later and follow up to future

Retrospective/historical = use health records to start follow up to identify outcomes
data collection starts in past

Question 3

Measurements cohort study

Answer 3

Incidence rates/rate ratio

Odds ratio

Question 4

Ideal study

Answer 4

Compare like with like - everything is the same apart from exposure of interest

Problem is this is virtually impossible - exposure is lined to other factors

Question 5

Ideal vs what we can use

Answer 5

Ideal = experiment
Next best = randomised control trial (cannot do this due to ethical reasons for smoking for example)
Next best = cohort study

Question 6

Person years calculation

Answer 6

Multiply people studies by the time they were studied for (if all timings same)

If not add up all years each person was studied for

Question 7

Incidence rate =

Answer 7

diseased / person years

diseased in unexposed / person years

Question 8

Incidence rate ratio =

Answer 8

Incidence rate of diseased / incidence rate of diseased in unexposed

Question 9

incidence rate example

Answer 9

If you followed 3 people for 10 years, person years = 3x10 = 30
if 2 people got asthma it would be 2/30 for incidence rate

Question 10

Calculating error factor

Answer 10

e to the power of 2x square root of 1/diseased unexposed + 1/diseased exposed

Question 11

Confidence interval

Answer 11

Incidence rate ratio / EF (lower limit)

Incidence rate ratio x EF (upper limit)

Question 12

Advantages of cohort study

Answer 12

Study exposures and personal characteristics (that are not usually collected)
Obtain more detailed info on outcomes and exposures
Collect additional data on potential confounders

Question 13

Exposure data can be

Answer 13

Binary - yes/no
Several categories
Continuous

Question 14

Comparisons can be made

Answer 14

Internally or externally in cohort study

Question 15

Internal comparison

Answer 15

Comparison of sub-cohorts

Calculate incidence rate ratio and error factor

Question 16

External comparison

Answer 16

Compare with external/reference population
Calculate standardised mortality ratio and error factor

= Observed number of cases / expected number of cases
e to the power of 2x square root of 1/Observed `

Question 17

Calculating expected cases

Answer 17

Cohort studies conducted over long time
people age as study continues
Disease rates in population change during study

Question 18

What is Lexis diagram?

Answer 18

Calculate expected number of deaths for each age group in each calendar time period

Question 19

Sample size and error factor relationship

Answer 19

As sample size increases, error factor decreases (results more reliable/precise)

Question 20

Limitations of external comparison

Answer 20

Useful when cannot use sub-cohorts 
But limited data is available 
no incidence data
Use mortality rates instead 
SELECTION BIAS?
SMR well below 100%

Question 21

What will standardised mortality ratio usually be

Answer 21

Well below 100% due to healthy worker effect - employment is restricted to healthy individuals so not representative of external population

Question 22

Advantages of concurrent cohort studies

Answer 22

detailed assessment of exposure, outcome and confounders

historical not as good at this

Question 23

Advantage of cohort over case control

Answer 23

study a range of outcomes (not restricted)
good for studying rare exposure
establish exposure that precedes outcome
good for conditions that fluctuate with age

(NOT AS GOOD FOR RARE OUTCOMES - they just wont happen)

Question 24

Disadvantages cohort study

Answer 24

Long time - expensive
Definitions of exposure and outcome = detailed
Risk of loss (survivor bias - those that are left are different)
Results = long time
Not good for rare outcomes
Difficulty with confounding

Question 25

Comparison of cohort studies

Answer 25

Internal - incidence rate ratio

External - standardised mortality ratio

Question 26

Cohort studies issues

Answer 26

Loss to follow up (survivor bias/differential loss)
Information bias (differential/non differential misclassification)
Confounding
Chance - random error