Each case and his/her matched controls is identified by a unique value of the match variable. 10 controls were selected for each case.  To be elegible for selection, a control had to be the same age as the case and be  in the cohort on the date of the case's diagnosis. Both cases and controls had to have at least one year of follow-up at this time.  See Talbot et al. (N Engl J Med 2005; 352:2082-90).