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Non-technical summaryInvestment in continuing vocational training constitutes a major part of post school human capital increment. Obviously, heterogeneity of employees plays a role not only in obtaining skills, but also in economic consequences of education and training.Empirical work on the wage impact of training has noted that unobserved heterogeneity of training participants should play a role. The expected return to training, which partly depends on unobservable characteristics, is likely to be a crucial criterion in the decision to take part in training or not. Therefore, the decision to take part in continuous training is likely to be influenced by the expected returns to training; i.e. those workers for whom the expected return is higher will obtain more training than other workers for whom the expected return is lower. Hence, participants and nonparticipants in training are unlikely to have the same observed and hypothetical returns. Severe econometric problems are therefore posed by the endogeneity of training decisions.While former empirical work with German data has extensively analyzed the wage effect of training, none of them has accounted for the likely possibility that worker selection into training is based on the expected heterogeneous return to continuous training. We try to account for this by using recent advances in estimating returns to schooling, which allow for selection on unobservables, and apply it to estimating the impact of training on earnings.Allowing heterogeneity to be unobserved by the econometrician, but assuming that individuals may act upon this heterogeneity, completely changes the interpretation and properties of commonly used estimators.We use German survey data from 1998/1999 and the Local IV method, which allows for observed as well as unobserved heterogeneity and that selection into training may depend on both. Our LIV estimate is much lower than the relevant OLS and IV estimate (and furthermore, insignificant). We cannot find any causal effect of training on wages when taking into account that more able and motivated individuals participate in training, or those which are on a promotion path where training courses are part of the way. 1 We thank Alexandra Spitz, Thomas Zwick, and seminar participants in Mannheim for helpful comments. We also thank Iliyan Stankov for his research assistance. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through the resear...