Characteristics of non-participants and participants.

Abbreviation: n/a, not applicable.aall non-participants vs. all participants; p-values calculated from chi-square statistics.brelapsed vs. non-relapsed participants; p-values calculated from chi-square statistics.cincludes high school, teachers training colleges, technical colleges and upper vocational education.dwithout radiotherapy, may have surgery.ewith or without chemotherapy or surgery.