The validity of instrumental variables (IV) regression models depends crucially on fundamentally untestable exclusion restrictions. Typically exclusion restrictions are assumed to hold exactly in the relevant population, yet in many empirical applications there are reasonable prior grounds to doubt their literal truth. In this paper I show how to incorporate prior uncertainty about the validity of the exclusion restriction into linear IV models, and explore the consequences for inference. In particular I provide a mapping from prior uncertainty about the exclusion restriction into increased uncertainty about parameters of interest. Moderate prior uncertainty about exclusion restrictions can lead to a substantial loss of precision in estimates of structural parameters. This loss of precision is relatively more important in situations where IV estimates appear to be more precise, for example in larger samples or with stronger instruments. The author illustrates these points using several prominent recent empirical papers that use linear IV models.
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Edward L. Glaeser & Rafael La Porta & Florencio Lopez-de-Silanes & Andrei Shleifer, 2004.
"Do Institutions Cause Growth?,"
Journal of Economic Growth,
Springer, vol. 9(3), pages 271-303, 09.
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Edward L. Glaeser & Rafael La Porta & Florencio Lopez-de-Silane & Andrei Shleifer, 2004.
"Do Institutions Cause Growth?,"
NBER Working Papers
10568, National Bureau of Economic Research, Inc.
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