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3420286 
Journal Article 
Instruments for causal inference: An epidemiologist's dream? 
Hernán, MA; Robins, JM 
2006 
Yes 
Epidemiology
ISSN: 1044-3983
EISSN: 1531-5487 
17 
4 
360-372 
English 
16755261 
WOS:000238740600004 
has comment/response 3420287 Instrumental variable analysis of secondary pharmacoepidemiologic data
has erratum 3420289 : Erratum
The use of instrumental variable (IV) methods is attractive because, even in the presence of unmeasured confounding, such methods may consistently estimate the average causal effect of an exposure on an outcome. However, for this consistent estimation to be achieved, several strong conditions must hold. We review the definition of an instrumental variable, describe the conditions required to obtain consistent estimates of causal effects, and explore their implications in the context of a recent application of the instrumental variables approach. We also present (1) a description of the connection between 4 causal models-counterfactuals, causal directed acyclic graphs, nonparametric structural equation models, and linear structural equation models-that have been used to describe instrumental variables methods; (2) a unified presentation of IV methods for the average causal effect in the study population through structural mean models; and (3) a discussion and new extensions of instrumental variables methods based on assumptions of monotonicity.