The use of statistics to examine the association between fluoride in drinking water and cancer death rates

"The basis of statistical tests of significance of association between fluoride level in drinking water and cancer death rates is discussed. Reference is made to two reported studies in each of which cancer death rates of a number of [U.S.] cities were used. It is argued that between city variation should be taken into account when performing tests of significance. In one of the two studies this was done informally; in the other between city variation was ignored."