Actually, all statistics about the IQ or health of breast-feed babies and formula-feed babies are the average figures. I met some breast-feed babies who often got sick and I also knew some very strong formula-feed babies. There is no way to pick up any two babies and make a direct comparison.
Every mother has her own choice on how to feed or what to feed her babies. Many mothers feel proud to breast feed her babies but they shouldn't feel superior over other mommies who are not doing the same.