**Research Papers**

Heterogeneous Coefficients, Control Variables, and Identification of Multiple Treatment Effects (with Whitney Newey).

Accepted for publication at * Biometrika*.

We use control variables to give necessary and sufficient conditions for identification of average treatment effects. With mutually exclusive treatments we find that, provided the heterogeneous coefficients are mean independent from treatments given the controls, a simple identification condition is that the generalized propensity scores (Imbens, 2000) be bounded away from zero and that their sum be bounded away from one, with probability one. Our analysis extends to distributional and quantile treatment effects, as well as corresponding treatment effects on the treated. These results generalize the classical identification result of Rosenbaum and Rubin (1983) for binary treatments.

Control Variables, Discrete Instruments, and Identification of Structural Functions (2021, with Whitney Newey).* Journal of Econometrics*, 222(1), Part A, pp. 73-88

**.**First arXiv version September 2018.

Control variables provide an important means of controlling for endogeneity in econometric models with nonseparable and/or multidimensional heterogeneity. We allow for discrete instruments, giving identification results under a variety of restrictions on the way the endogenous variable and the control variables affect the outcome. We consider many structural objects of interest, such as average or quantile treatment effects.

Gaussian Transforms Modeling and the Estimation of Distributional Regression Functions (2020, with Richard Spady).

Revise and resubmit at * Econometrica*.

We propose flexible Gaussian representations for conditional distribution functions and give a concave likelihood formulation for their global estimation. We obtain solutions that satisfy the monotonicity property of conditional distribution functions, including under general misspecification and in finite samples. A Lasso-type penalized version of the corresponding maximum likelihood estimator is given. Inference and estimation results for conditional distribution, quantile and density functions are provided.

Supplementary material.

Econometric Society World Congress 2020 presentation.

Semiparametric Estimation of Structural Functions in Nonseparable Triangular Models (2020, with Victor Chernozhukov, Iván Fernández-Val, Whitney Newey, and Francis Vella).* Quantitative Economics, *11(2), pp. 503–533. First arXiv version November 2017.

We introduce two classes of semiparametric nonseparable triangular models that are based on distribution and quantile regression modelling of the reduced form conditional distributions of the endogenous variables. We show that average, distribution and quantile structural functions are identified through a control function approach that does not require a large support condition. We propose a computationally attractive three-stage procedure to estimate the structural functions. We provide asymptotic theory and uniform inference methods for each stage.

Heterogenous Coefficients, Discrete Instruments, and Identification of Treatment Effects (2018, with Whitney Newey). *arXiv*.

We consider heterogenous coefficients models
where the outcome is a linear combination of known functions of treatment and
heterogenous coefficients. We use control variables to obtain identification results
for average treatment effects. With discrete instruments in a triangular model we
find that average treatment effects cannot be identified when the number of support
points is less than or equal to the number of coefficients. A sufficient condition for
identification is that the second moment matrix of the treatment functions given the
control is nonsingular with probability one.

Simultaneous Mean-Variance Regression (2018, with Richard Spady). *arXiv*. Supplementary material.

We propose simultaneous mean-variance regression for the linear estimation and approximation of conditional mean functions. In the presence of
heteroskedasticity of unknown form, our method accounts for varying dispersion
in the regression outcome across the support of conditioning variables. Simultaneity generates outcome predictions that are guaranteed to improve over OLS prediction error, with corresponding parameter standard errors that
are automatically valid. Under misspecification, the corresponding mean-variance regression model weakly dominates the OLS location model under a
Kullback-Leibler measure of divergence, with strict improvement in the presence of
heteroskedasticity. We obtain large improvements
over OLS and WLS in terms of estimation and inference in
finite samples.

Dual Regression (2018, with Richard Spady).* Biometrika*, 105(1), pp. 1–18 (lead article). First arXiv version October 2012.

We propose dual regression as an alternative to the quantile regression process for the global estimation of conditional distribution functions under minimal assumptions. Dual regression provides all the interpretational power of the quantile regression process while avoiding the need for repairing the intersecting conditional quantile surfaces. Our approach introduces a mathematical programming characterization of conditional distribution functions. We apply our general characterization to the specification and estimation of a flexible class of conditional distribution functions, and present asymptotic theory for the corresponding empirical dual regression process.

Cross-Validation Selection of Regularization Parameter(s) for Semiparametric Transformation Models (2017, with Senay Sokullu).
* Annals of Economics and Statistics*, 128, pp. 67–108.

We propose cross-validation criteria for the selection of regularisation parameter(s) in the semiparametric instrumental variable transformation model proposed in Florens and Sokullu (2016). This model is characterized by the need to choose two regularisation parameters, one for the structural function and one for the transformation of the outcome. We consider two-step and simultaneous criteria, and analyze the finite-sample performance of the estimator using the corresponding regularisation parameters. We find that simultaneous selection of regularisation parameters provides significant improvements in the performance of the estimator.

**Research Grants**

Principal Investigator for the ESRC** New Investigator** Grant (2019-2022):

“Policy Evaluation Beyond Averages: Distributional Impact Analysis”.

**Official Reports**

Lavender, T., Wilkinson, L., Ramsay, G., Stouli, S., Adshead, S., & Chan, Y. S. (2020), “Research into Recent Dynamics of the Press Sector in the UK and Globally“, prepared for the Department for Digital, Culture, Media & Sport (DCMS).

Guseva, M., Nakaa, M., Novel, A. S., M., Pekkala, K., Souberou, B., & Stouli, S. (2008), “Press Freedom and Development”, UNESCO.

Contribution to the Maghreb database of policy reform indicators in the annex of World Bank (2006), “Is There a New Vision for Maghreb Economic Integration?”, Volume 1. Main Report. Washington, DC.