× 9 combinations of residence-gender-age categories). To select controls, a random number was assigned to each subject using statistical software, and the records were sorted based on this random number. Each subject with a number below the target enrollment number for the individual's cell was selected to be invited into the study. The family physicians for the potential study subjects were identified. The selected physicians were then asked to provide information about eligibility of the potential study subjects and, if eligible, to contact the selected controls to inform them about the study. Eligible controls were contacted by letter from the study personnel, followed by a phone call. When the phone number for the selected individuals could not be found, we searched for the phone numbers of other members of the family identified through contact with the corresponding municipalities, or sent pre-stamped return-cards requesting contact information.
