Instrumentality Tests Revisited
This work addresses the problem of identifying valid instruments in econometrics and social sciences, offering incremental improvements to existing methods.
The paper studied Pearl's necessary test for instrumental variables in linear models with correlated errors, providing a novel interpretation, a general theory of instrumental tests, and new tests for discrete and continuous variables.
An instrument is a random variable thatallows the identification of parameters inlinear models when the error terms arenot uncorrelated.It is a popular method used in economicsand the social sciences that reduces theproblem of identification to the problemof finding the appropriate instruments.Few years ago, Pearl introduced a necessarytest for instruments that allows the researcher to discard those candidatesthat fail the test.In this paper, we make a detailed study of Pearl's test and the general model forinstruments. The results of this studyinclude a novel interpretation of Pearl'stest, a general theory of instrumentaltests, and an affirmative answer to aprevious conjecture. We also presentnew instrumentality tests for the casesof discrete and continuous variables.