Assumption 5 at the top of page 6 was incorrectly published with the “=s” as a subindex in the conditional expectations. It should be as follows:

“ ${E}({Y} _ { i0} \mid{\bf S} _ { {i}} ={\rm s}) \ {\rm and} \ {E(Y _ { i1} \mid {\bf S} _ { i} }={\rm s)}$ are continuous in s at all points b on the boundary.”