Criteria for grading the internal validity of individual studies
systematic review
case-control study
RCTs and cohort studies
diagnostic accuracy studies
any systematic error in the design, conduct or analysis of a study that results in a mistaken estimate of an exposure's effect on the risk of disease
bias
Confounding
third factor
When the incidence rate of disease in the presence of two or more risk factors differs from the incidence rate exposed to result from their individual effect
interaction
3 questions to determine if interaction is present
is there association?
if so, is it due to confounding?
is the association equally strong in strata formed on the basis of a third variable?
No = interaction present Yes = interaction not present