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Constructing an Instrument as a Function of Covariates

Moses Stewart

arXiv 13 Mar 2025 · Econometrics

arXiv:2503.10929 · PDF · Extracted main text

Abstract

Researchers often use instrumental variables (IV) models to investigate the causal relationship between an endogenous variable and an outcome while controlling for covariates. When an exogenous variable is unavailable to serve as the instrument for an endogenous treatment, a recurring empirical practice is to construct one from a nonlinear transformation of the covariates. We investigate how reliable these estimates are under mild forms of misspecification. Our main result shows that for instruments constructed from covariates, the IV estimand can be arbitrarily biased under mild forms of misspecification, even when imposing constant linear treatment effects. We perform a semi-synthetic exercise by calibrating data to alternative models proposed in the literature and estimating the average treatment effect. Our results show that IV specifications that use instruments constructed from covariates are non-robust to nonlinearity in the true structural function.

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Most heavily cited references

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ReferenceIntensityMentionsSectionsMain text
1Aladangady, A (2017) Housing Wealth and Consumption: Evidence from Geographically-Linked Microdata1.00083100%
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3Liu, H., Liu, L., and Wang, F (2023) Housing wealth and fertility: evidence from China0.81142100%
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5Disney, R., Gathergood, J., and Henley, A (2010) House Price Shocks, Negative Equity, and Household Consumption in the United Kingdom0.7375260%
6Andrews, I., Barahona, N., Gentzkow, M., Rambachan, A., and Shapiro,… (2023) Structural Estimation Under Misspecification: Theory and Implications for Practice0.73732100%
7Gao, W. Y. and Wang, R (2023) IV Regressions without Exclusion Restrictions0.73732100%
8Blandhol, C., Bonney, J., Mogstad, M., and Torgovitsky, A (2022) When is TSLS Actually LATE?0.6938250%
9Bettinger, E. P., Fox, L., Loeb, S., and Taylor, E. S (2017) Virtual Classrooms: How Online College Courses Affect Student Success0.64422100%
10Rogall, T (2021) Mobilizing the Masses for Genocide0.64422100%

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