Breast cancer researchers found that, out of a sample of 1000 women who had been treated for breast cancer, over 60% had had a miscarriage in their lifetime. This led the researchers to conclude that there is a connection between breast cancer and miscarriages, and to encourage all women who have had miscarriages to be screened for breast cancer more frequently than women who have not had miscarriages.
Which of the following, if true, most weakens the researchers' conclusions?