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In epidemiological research, the unavailability of possible confounders may lead to inconsistent estimates when a randomized experimental design is not feasible. A related challenge can be possible reverse causal orders between the dependent and the independent variables. This is a so-called endogeneity problem, which is well known in econometrics. 1 However, if possible confounders are unavailable, or we suspect reverse causal orders, the appropriate use of instrumental variables (IVs) can generate consistent estimates. IVs have the properties that they are correlated with the explanatory variables and uncorrelated

Instrumental variables in epidemiological research: an assessment of the adoption rate and future trends