Transcript Case-control study 1: Design and odds ratio Preben Aavitsland (partly based on Epiet 2004)

Case-control study 1:
Design and odds ratio
Preben Aavitsland
(partly based on Epiet 2004)
Contents
• Monday 1
– Design: Case-control study as a smarter cohort study
– The odds ratio
• Tuesday 2
– Choosing cases and controls
– Matching
– Power calculation
• Wednesday
– Case-control studies in outbreaks
• Thursday 3
– Bias and confounding
– Analysis
Why case-control study
• Best way to acquire knowledge about
causes and protective factors of disease
• Both for outbreaks and endemic diseases
• Easy to perform and analyse
• Needs thorough planning
• Next step after surveillance and outbreak
investigations
Source population
Source population
Exposed
Unexposed
The cohort study
exposed
unexposed
The cohort study
exposed
Occurrence among
exposed (I1 or R1)
a
unexposed
b
Occurrence among
unexposed (I0 or R0)
The cohort study: calculation
Exposed
I1 = a / Nt1
= 16 / 125 pyar
Unexposed
I0 = b / Nt0
= 8 / 120 pyar
IRR =
I1 = a / Nt1
I0
b / Nt0
= 16/125 pyar = 1.92
8/120 pyar
Problems of cohort
• Large sample size required
– At least if disease is rare
•
•
•
•
•
•
•
Latency period
Time consuming
Loss to follow up
Changing exposure over time
Only one exposure
Ethical considerations
Cost
That is why
the case-control study is smarter
• Gives the same measure of causal effect
as the cohort study (risk ratio RR or
incidence rate ratio IRR)
• It is called odds ratio (OR)
• Easier, quicker, cheaper, smarter
Source population
Exposed
Unexposed
The case-control study
Source population
Exposed
Cases
Unexposed
Source population
Exposed
Sample
Unexposed
Controls
Cases
Source population
Exposed
Sample
Cases
Unexposed
Cases = the same as in cohort
study
Controls = sample of the source
population, with representative
distribution of exposed and
unexposed persons (or persontime)
Controls
Source population
a
b
Exposed
Sample
Cases
Unexposed
c
d
Controls
Source population
a
b
Exposed
Sample
Cases
Unexposed
c
d / Nt0 = c / Nt1
d
because sampled
independent of
exposure
24 / 120 = 25 / 125
Controls
The case-control study: calculation
IRR = I1 = a / Nt1
I0 b / Nt0
d= c
Nt0 Nt1
= a . Nt0
b Nt1
Nt0 = d
Nt1 c
=a.d
b c
= 16 . 24 = 1.92
25 8
Saving resources with case-control study
• In stead of following a cohort of 245 people for
one year to wait for the 24 cases
• We investigated the 24 cases in order to divide
them between
– exposed a = 16 and
– unexposed b = 8
• We chose 49 controls and investigated them in
order to divide them between
– exposed c = 25 and
– unexposed d = 24
• The result
– exactly the same as cohort study, but much easier
The odds ratio (OR)
• OR = Incidence rate ratio (IRR)
• OR = Risk ratio (RR)
Exposed
Unexposed
Cases
a
b
Controls
c
d
• Cross product ratio: ad / bc
Summary of the case-control study
• Imagine a source population
– Consists of exposed and unexposed people
– Gives rise to cases (same as if cohort study)
• Control group is a sample from this source
population
– Independent of exposure status
– Same distribution of exposed persons (person-time) as
in source population
• Determine exposure status of cases and controls
• Calculate odds ratio
– = risk ratio or incidence rate ratio if a cohort had been
done
Challenges in case control study
• No measure of disease occurrence
– Not risk R or incidence rate IR
• Difficult to define source population
• Difficult to sample controls correctly
– Independently from exposure
• Recall bias
– Cases remember differently from controls