A certain scientist concluded his study in fertility control as follows: "So far one result has emerged from the before-and-after survey, and it is a key measure of the outcome: at the end of 1962, 14.2% of the women in the sample were pregnant, and at the end of 1963 (after the birth-control campaign) 11.4 % of the women (in a second independent sample) were pregnant, a decline of about one fifth." If the samples (both before and after) included 2500 women, what statistical qualification would you add to the above statement, in order to make its meaning clearer?