Data have been given on N=4642 infant births and we are asked to examine factors that determine the outcome of a low birthweight baby. There are 2 probit models (Model 1 and Model 2) with the dependent variable LBWEIGHT = 1 if it is a low birthweight baby and 0 otherwise. MAGE is the mother's age, PRENATAL1 = 1 if the first prenatal visit is in the first trimester and 0 otherwise, and MBSMOKE = 1 if the mother smoked and 0 otherwise. The results are in the table below (C refers to a constant and standard errors are in parentheses).
1) How can we comment on the estimated signs and significance of the coefficients on PRENATAL1 and MBSMOKE in Model 1?
2) Using Model 1, how do we calculate the marginal effect on the probability of a low birthweight baby given an increase in the mother's age by 1 year, for a woman who is 20 years old with PRENATAL1 = 0 and MBSMOKE = 0? If this calculation is repeated for a woman who is 50 years old, do the results make sense?
3) Using Model 2, how do we calculate the impact of a prenatal visit in the first trimester on the probability of having a low birthweight baby for a woman who is 30 years old and smokes?