Imbens, Guido W.; Rosenbaum, Paul R. - In: Journal of the Royal Statistical Society Series A 168 (2005) 1, pp. 109-126
An instrument or instrumental variable manipulates a treatment and affects the outcome only indirectly through its manipulation of the treatment. For instance, encouragement to exercise might increase cardiovascular fitness, but only indirectly to the extent that it increases exercise. If...
