Sebastian Kripfganz

Lecturer (assistant professor) in Economics,
University of Exeter Business School, Department of Economics

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Title and Abstract
kinkyreg: Instrument-free inference for linear regression models with endogenous regressors
In models with endogenous regressors, a standard regression approach is to exploit just-identifying or overidentifying orthogonality conditions by using instrumental variables. In just-identified models, the identifying orthogonality assumptions cannot be tested without the imposition of other nontestable assumptions. While formal testing of overidentifying restrictions is possible, its interpretation still hinges on the validity of an initial set of untestable just-identifying orthogonality conditions. We present the kinkyreg command for kinky least-squares inference, which adopts an alternative approach to identification. By exploiting nonorthogonality conditions in the form of bounds on the admissible degree of endogeneity, feasible test procedures can be constructed that do not require instrumental variables. The kinky least-squares confidence bands can be more informative than confidence intervals obtained from instrumental-variables estimation, especially when the instruments are weak. Moreover, the approach facilitates a sensitivity analysis for standard instrumental-variables inference. In particular, it allows the user to assess the validity of previously untestable just-identifying exclusion restrictions. Further instrument-free tests include linear hypotheses, functional form, heteroskedasticity, and serial correlation tests.
Suggested Citation
Kripfganz, S., and J. F. Kiviet (2021). kinkyreg: Instrument-free inference for linear regression models with endogenous regressors. Stata Journal 21 (3), 772-813.
Related Work
Kiviet, J. F., and S. Kripfganz (2021). Instrument approval by the Sargan test and its consequences for coefficient estimation. Economics Letters 205, 109935.
Authors
Sebastian Kripfganz
University of Exeter
Jan F. Kiviet
University of Amsterdam;
Stellenbosch University
Journal Article
Stata Journal 21 (3), 772-813
Article DOI
10.1177/1536867X211045575
Stata Program
kinkyreg

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