Identifying effect modification stratification lung

Identifying Effect modification: Stratification
Lung
cancer
Yes
Lung
cancer
No
Alcohol
Yes
180
(A)
120
(B)
No alcohol
No
120
(C)
180
(D)
Lung Ca
Smokers
Lung Ca
Non-smokers
Yes No Yes No
Alcohol
Yes
140 20 40 100
No alcohol
No
70 50 50 130
Crude OR=AD/BC
=180x180/120x120
= 2.25
OR among smokers
=140x50/70x20
=5
OR among non-smokers
=40x130/100x50
=1
Crude OR Stratified OR

Interpretation
 Crude analysis – alcohol was associated
with an  risk of lung cancer
 Stratified analyses

Alcohol consumption was associated with an

risk of lung cancer among smokers , but not
among non-smokers

Smoking modifies the effect of alcohol
consumption on lung cancer risk
 Stratum specific results must be
presented if effect modification is present

Confounding vs. Effect Modification

Confounding
•
Nuisance that you must get rid of (biases the
occurrence relation)
•
Similar effect in different strata
 Effect modification
•
Important aspect of the occurrence relation
and must be understood in order to draw
specific conclusions
•
Different effects in different strata

Confounding vs. Effect modification
 Compare crude effect estimates with stratum-
specific effect estimates
All Men Women Confounding EM
cRR=4.5 RR=1.0 RR=1.0 + -
cRR=5.5 RR=9.0 RR=1.5 (-) +
cRR=1.0 RR=4.0 RR=4.0 + -
cRR=4.0 RR=4.0 RR=4.0 - -

Framework for identifying confounding and
Effect Modification
Yes
Yes
No
N o
Yes
Is there an association
between an exposure and
outcome?
Is it due to confounding
or bias
Is the association of similar
magnitude in subgroups of
the population
Spurious association
Effect modification
present
No effect modification
Adapted from Szklo M and Nieto J. Epidemiology: Beyond the basics. Second edition

Framework – Confounding vs. EM II
R e p o r t c r u d e O R / R R
N o c o n f o u n d i n g
S t r a t a O R / R R s i m i l a r t o c r u d e
A d j u s t , a n d p r e s e n t a d j u s t e d O R / R R
C o n f o u n d i n g p r e s e n t
S t r a t a O R / R R d i ff e r e n t f r o m c r u d e
I d e n t i c a l O R / R R a c r o s s s t r a t a
R e p o r t O R / R R
f o r e a c h s t r a t u m
E ff e c t m o d i fi c a t i o n
D i ff e r e n t O R / R R a c r o s s s t r a t a
E x a m i n e O R s / R R s i n e a c h s t r a t u m
E x a m i n e c r u d e O R / R R

 “The world is richer in associations than meanings, and
it is the part of wisdom to differentiate the two” – John
Barth

Questions?