Session 7 Lecture 1 : Cohort studies Flashcards
How to conduct a cohort study?
- identify outcome-free individuals
- group individuals according to the level of exposure
- follow up over time until you discover outcome for everyone
- analysis can be incidence rates or odds ratio
- compare rates for each exposure groups
How do you calculate the incidence rate?
Number of new cases / patient time at risk
- patient time at risk = ( number of patients X time at risk )
often reported as a rate: events per person per year as you multiply by the answer by 1000 (or any multiple of 10) to get rid of decimals so answers would be in ‘per 1000 person-year’
How do you calculate the incidence rate ratio?
IRR = incidence rate in group A / incidence rate in group B
Group a= exposed
Group b = non exposed
- used to compare exposure groups
How do you make an internal comparison of a cohort study?
Sub cohorts - exposed and unexposed
- Calculate the Incidence rate ratio
- Calculate the error factor
- Calculate the Confidence Interval
How do you make an external comparison of a cohort study against an external reference population?
- Calculate Standardised Mortality Ratio
Observed number of cases / Expected number of cases - Calculate the error factor
- Calculate Confidence Interval
What are the limitations of External Comparison in Cohort studies?
- limited data available for the reference population
- study and reference populations may not be comparable; selection bias
What is a prospective or concurrent cohort study?
- involves prospective follow up
- recruits outcome free individuals and classify them according to their exposure status
- follow up immediately or later
e. g. study starts in 2020, follow up immediately up until 2030 or follow up from 2023-2030
What is a historical or “retrospective” cohort study?
- follow up data collected from the past
- recruit outcome free individuals
- classify their initial exposure status and subsequent outcome status using historical records
e. g. study starts in 2020, follow up from 2006 onwards
What are the advantages of cohort studies over routinely available data?
- can study exposures and personal characteristics that are not routinely collected
- can obtain more detailed information on outcomes and exposures
- can collect additional data on potential confounding factors
What are the advantages of Cohort studies?
- good for rare exposures
- can measure multiple outcomes
- allows for a temporal sequence to be determined (there is a time order of the events)
What are the advantages of prospective cohort studies over historical cohort studies?
enables detailed and prospective assessment of exposure, outcomes and confounders
What are the advantages of cohort studies over Case-control studies?
- can study a range of different outcomes
- good at studying rare exposures
- good at establishing that exposures precedes outcomes
What are the disadvantages of Cohort studies?
- not good for rare outcomes (case-control good for rare outcomes)
- takes a long time for the study to be done
- risk of a high number of losses to follow up
- results take a long time
A Worked Example
- 1000 children followed up from birth to age 5 yrs
- 300 children had at least one parent smoke at home
- 700 children had neither parent smoke at home
Calculate the persons years at risk?
number of patient (person) X time at risk
- p-y smoke = 300 X 5= 1500
- p-y non smoke = 700 X 5 = 3500
A Worked Example
- 1000 children followed up from birth to age 5 yrs
- 300 children had at least one parent smoke at home
a) of the 300 children, 75 developed asthma - 700 children had neither parent smoke at home
a) of the 700 children, 105 developed asthma
Given that the patient time at risk for the smoker’s group is 1500 and the non-smokers group is 3500, calculate the Incidence ratio.
Number of new cases / patient time at risk
- IR smoke = 75/1500 = 0.05
0. 05 X 1000 = 50 per 1000 p-y - IR non smoke = 105/3500 = 0.03
0. 03 X 1000 = 30 per 1000 p-y
