... variety of sample selection issues, including tests and corrections Section 17. 7 treats sample selection and the related problem of attrition in panel data Stratified sampling, which arises out of ... outcome of a Tobit, rather than a probit, equation The analysis of the models in this section comes from Wooldridge (1998) The model in Section 17. 5.1 is a special case of the model studied by Vella ... We explicitly consider the case of a single endogenous explanatory variable, as in Section 17. 4.2 We use equations (17. 25) and (17. 26), and, in place of equation (17. 27), we have a Tobit selection...