The log-likelihood for the Heckman regression-selection model is given in Equation XXXXXXXXXXpage 634). Derive this expression. (Hint: The first sum in the log-likelihood, for the observations for...

The log-likelihood for the Heckman regression-selection model is given in Equation 20.20 (page 634). Derive this expression. (Hint: The first sum in the log-likelihood, for the observations for which Y is missing, is of the log-probability that each such Yi is missing; the second sum is of the log of the probability density at the observed values of Yi times the probability that each such value is observed.)