  Instrumental variables are widely used for estimating causal effects in the
presence of unmeasured confounding. The discrete instrumental variable model
has testable implications on the law of the observed data. However, current
assessments of instrumental validity are typically based solely on
subject-matter arguments rather than these testable implications, partly due to
a lack of formal statistical tests with known properties. In this paper, we
develop simple procedures for testing the binary instrumental variable model.
Our methods are based on existing approaches for comparing two treatments, such
as the t-test and the Gail--Simon test. We illustrate the importance of testing
the instrumental variable model by evaluating the exogeneity of college
proximity using the National Longitudinal Survey of Young Men.
