The widespread addition of fluoride to tap water in the United States and other countries has been credited with a drastic decrease in tooth decay. However, there is always some concern over possible harmful effects of any kind of treatment. Most studies so far have failed to show a negative impact of water fluoridation on health, but one study did report a significant decrease in IQ among individuals exposed to extra fluoride as children compared with individuals not exposed to this treatment. Commenting on this study in a news article on National Public Radio, Dr. Myron Allukian of the Harvard School of Dental Medicine said that the effect on IQ was small accounting for only about a half-point difference. He further explained that a half-point difference in IQ is meaningless, just like a difference in adult heights of half a millimeter.

a. Why is it important to know the size of an effect and not just that the effect was statistically significant?
b. How can an effect be both small and statistically significant?