Table 1. Characteristics of participants in each survey wave (base: individual twins).
| Characteristic (n) | Wave 1 (2818) | Wave 2 (2242) | Wave 3 (1464) | Wave 4 (1516) |
|---|---|---|---|---|
| Average age | 14.91 | 18.00 | 22.51 | 30.75 |
| Twin type | ||||
| MZ (pairs) | 1310 (655) | 1076 (538) | 752 (376) | 746 (373) |
| DZS (pairs) | 824 (412) | 652 (326) | 406 (203) | 436 (218) |
| DZO (pairs) | 684 (342) | 514 (257) | 306 (153) | 334 (167) |
| Gender | ||||
| Males (%) | 46 | 44 | 41 | 41 |
| Females (%) | 54 | 56 | 59 | 59 |
| Smoking status | ||||
| Never smoked (%) | 37 | 23 | 22 | 13 |
| Experimenter (%) | 49 | 54 | 49 | 55 |
| Current smoker (%) | 14 | 23 | 29 | 20 |
| Ex-smoker (%) | NA | NA | NA | 11 |
| Friend's smoking | ||||
| No friends smoke (%) | 65 | 44 | 39 | 46 |
| <50% smoking (%) | 20 | 32 | 42 | 39 |
| ⩾50% smoking (%) | 15 | 25 | 19 | 16 |
| Parent's current smoking | ||||
| None (%) | 65 | 69 | 77 | 77 |
| One parent (%) | 26 | 24 | 19 | 18 |
| Both parents (%) | 10 | 7 | 4 | 5 |
| Meet with twin | ||||
| Everyday (%) | 96 | 81 | 46 | 11 |
| Weekly (%) | 2 | 10 | 22 | 32 |
| Less than weekly (%) | 2 | 10 | 33 | 57 |
DZO=opposite-sex dizygotic or fraternal twins; DZS=same-sex dizygotic or fraternal twins; MZ=monozygotic or identical twins; NA=not applicable.
NB: % may not add to 100 due to rounding.