Confounding and effect modification. Preben Aavitsland. Can we believe the result?. Rice. Salmonellosis. OR = 3.9. Systematic error. Does not decrease with increasing sample size Selection bias Information bias Confounding. Confunding - 1.
OR = 3.9
“Mixing of the effect of the exposure on disease with the effect of another factor that is associated with the exposure.”
Post op inf.
Op theatre I
1 A confounder must be a cause of the disease (or a marker for a cause)
2 A confounder must be associated with the exposure in the source population
3 A confounder must not be affected by the exposure or the disease
“Second, third and fourth child are more often affected by Downs’ syndrome.”
”The Norwegian comedian Marve Fleksnes once stated: I am probably allergic to leather because every time I go to bed with my shoes on, I wake up with a headache the next morning.”
“A study has found that small hospitals have lower rates of nosocomial infections than the large university hospitals. The local politicians use this as an argument for the higher quality of local hospitals.”
Restriction of the study or the analysis to a subgroup that is homogenous for the possible confounder.
Always possible, but reduces the size of the study.
We study only mothers of a certain age
35 year old mothers
“Selection of controls to be identical to the cases with respect to distribution of one or more potential confounders.”
Is rice a confounder for the chicken salmonellosis association?
Stratify: Make one 2x2 table for rice-eaters and one for non-rice-eaters (e.g. in Episheet)
OR for chicken alone = ORmh for chicken ”controlled for rice”
Is chicken a confounder for the rice salmonellosis association?
Stratify: Make one 2x2 table for chicken-eaters and one for non-chicken-eaters (e.g. in Episheet)
OR for rice alone = ORmh for rice ”controlled for chicken”