Arpino, Bruno and Mealli, Fabrizia (2008): The specification of the propensity score in multilevel observational studies.

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Abstract
Propensity Score Matching (PSM) has become a popular approach to estimation of causal effects. It relies on the assumption that selection into a treatment can be explained purely in terms of observable characteristics (the “unconfoundedness assumption”) and on the property that balancing on the propensity score is equivalent to balancing on the observed covariates. Several applications in social sciences are characterized by a hierarchical structure of data: units at the first level (e.g., individuals) clustered into groups (e.g., provinces). In this paper we explore the use of multilevel models for the estimation of the propensity score for such hierarchical data when one or more relevant clusterlevel variables is unobserved. We compare this approach with alternative ones, like a single level model with cluster dummies. By using Monte Carlo evidence we show that multilevel specifications usually achieve reasonably good balancing in cluster level unobserved covariates and consequently reduce the omitted variable bias. This is also the case for the dummy model.
Item Type:  MPRA Paper 

Original Title:  The specification of the propensity score in multilevel observational studies 
Language:  English 
Keywords:  propensity score, multilevel studies, unconfoundedness, causal inference 
Subjects:  C  Mathematical and Quantitative Methods > C2  Single Equation Models ; Single Variables > C21  CrossSectional Models ; Spatial Models ; Treatment Effect Models ; Quantile Regressions C  Mathematical and Quantitative Methods > C0  General > C01  Econometrics 
Item ID:  17407 
Depositing User:  Bruno Arpino 
Date Deposited:  20 Sep 2009 10:35 
Last Modified:  22 Feb 2016 09:00 
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URI:  https://mpra.ub.unimuenchen.de/id/eprint/17407 