149
Causation versus Association
Adjustment
for the third variable in this case can produce a spurious association in this example
•Missing days off work is informative about MI incidence
among those who do not use
marijuana
–Among people missing work, marijuana users will have
lower incidence of MI
•The incidence of MI will likely be similar between
marijuana users and nonusers
who do not miss work
–The resulting interaction will seem to be an association
in an adjusted analysis
Days
off
work
Marijuana
MI