In 1945, there were 1,000 women who worked in a factory painting radium dials on watches. The

incidence of bone cancer in these women up to 1975 was compared with that of 1,000 women who worked as telephone operators in 1945. A total of 20 of the radium dial painters and 4 of the telephone operators developed bone cancer between 1945 and 1975.

What is the relative risk of developing bone cancer for radium dial workers?