The International Classification of Diseases (ICD) diagnosis and Office of Population Censuses and Surveys (OPCS) operation codes used for the case and exposure conditions in this study were as follows:
| Diagnoses | ICD 7 | ICD 8 | ICD 9 | ICD 10 |
|---|---|---|---|---|
| Case conditions | ||||
| Ulcerative colitis | 572.2 | 563.1 | 556 | K51 |
| Crohn's disease | 572.0 | 563.0 | 555.0–555.2, 555.9 | K50 |
| Exposures | ||||
| Appendicitis | 550–552 | 540–542 | 540–542 | K35–K37 |
| Operations | OPCS 1 | OPCS 2 | OPCS 3 | OPCS 4 |
|---|---|---|---|---|
| The control conditions used in this study were as follows, using the terminology of ICD9 and the OPCS Classification of Surgical Operations, Third Revision: sebaceous cyst; disorders of tooth development and eruption; diseases of hard tissues of teeth; deflected nasal septum; nasal polyps; varicose veins of lower extremities; haemorrhoids; acute respiratory infections; strabismus; cataract; inguinal hernia; diseases of nail; internal derangement of knee; bunion; contraceptive management; dilatation of cervix and curettage of uterus; head injuries; fracture of upper limb and lower limb; disloations, sprains, and strains; superficial injury and contusion; total hip replacement; total knee replacement. | ||||
| Appendicectomy | 441 | 441, 443, 444 | 441–444 | H01, H02 |
| Tonsillectomy | 260–264 | 230–236 | 230–236 | F34, F36 |