Let kids denote the number of children ever born to a woman, and let educ denote years of education for the woman. A simple model relating fertility to years of education is

kids = B0 + B1edu + u

where u is the unobserved error.

(a)What kinds of factors are contained in u ? List at least two factors.

(b)Are other factors described in (a) likely to be correlated with level of education? Explain clearly.

(c)Explain the zero conditional mean assumption using this model.