Characteristics of students who answered all main outcome and exposure questions compared with students who were excluded from the analysis
| Students excluded from analysis (N (%)) | Students included in analysis (N (%)) | Ever-smokers (N (%)) | Susceptible never-smokers (N (%)) | χ2 p Value (included–excluded) | |
|---|---|---|---|---|---|
| Total | 1109 (100) | 5376 (100) | 953 (100) | 1204 (100) | |
| Sex | |||||
| Male | 679 (61.2) | 2644 (49.2) | 464 (48.7) | 585 (48.6) | <0.001 |
| Female | 426 (38.4) | 2728 (50.7) | 487 (51.1) | 618 (51.3) | |
| Missing | 4 (0.4) | 4 (0.1) | 2 (0.2) | 1 (0.1) | |
| School year | |||||
| Year 7 | 303 (27.3) | 1402 (26.1) | 98 (10.3) | 275 (22.8) | 0.340 |
| Year 8 | 333 (30.0) | 1546 (28.8) | 193 (20.3) | 346 (28.7) | |
| Year 9 | 250 (22.5) | 1279 (23.8) | 299 (31.4) | 309 (25.7) | |
| Year 10 | 214 (19.3) | 1144 (21.3) | 361 (37.9) | 274 (22.8) | |
| Missing | 9 (0.8) | 5 (0.1) | 2 (0.2) | 0 (0) | |
| Perceived academic performance | |||||
| Excellent or good | 738 (66.5) | 4129 (76.8) | 579 (60.8) | 885 (73.5) | <0.001 |
| Average or below average | 301 (27.1) | 1196 (22.2) | 358 (37.6) | 315 (26.2) | |
| Missing | 70 (6.3) | 51 (0.9) | 16 (1.7) | 4 (0.3) | |
| Rebelliousness | |||||
| Low | 507 (45.7) | 2733 (50.8) | 233 (24.5) | 463 (38.5) | 0.375 |
| High | 495 (44.6) | 2510 (46.7) | 684 (71.8) | 708 (58.8) | |
| Missing | 107 (9.6) | 133 (2.5) | 36 (3.8) | 33(2.7) | |
| Parental smoking | |||||
| Neither | 713 (64.3) | 3424 (63.7) | 409 (42.9) | 740 (61.5) | 0.056 |
| At least one | 347 (31.3) | 1910 (35.5) | 533 (55.9) | 452 (37.5) | |
| Missing | 49 (4.4) | 42 (0.8) | 11 (1.2) | 12 (1) | |
| Sibling smoking | |||||
| None | 930 (83.9) | 4716 (87.7) | 679 (71.3) | 1056 (87.7) | 0.530 |
| At least one | 130 (11.7) | 618 (11.5) | 263 (27.6) | 136 (11.3) | |
| Missing | 49 (4.4) | 42 (0.8) | 11 (1.2) | 12 (1) | |
| Smoking in the home | |||||
| Not allowed | 845 (76.2) | 4132 (76.9) | 567 (59.5) | 891 (74) | 0.046 |
| Allowed | 201 (18.1) | 1165 (21.7) | 363 (38.1) | 292 (24.3) | |
| Missing | 63 (5.7) | 79 (1.5) | 23 (2.4) | 21 (1.7) | |
| Number of friends who smoke | |||||
| None | 451 (40.7) | 2230 (41.5) | 75 (7.9) | 343 (28.5) | 0.051 |
| 1 or 2 | 127 (11.5) | 755 (14.0) | 142 (14.9) | 234 (19.4) | |
| 3 or more | 204 (18.4) | 1142 (21.2) | 517 (54.3) | 321 (26.7) | |
| Not sure | 267 (24.1) | 1203 (22.4) | 206 (21.6) | 298 (24.8) | |
| Missing | 60 (5.4) | 46 (0.9) | 13 (1.4) | 8 (0.7) | |
| Perceived peer smoking prevalence | |||||
| Underestimate or about right | 217 (19.6) | 985 (18.3) | 79 (8.3) | 193 (16) | 0.022 |
| Overestimate | 760 (68.5) | 4187 (77.9) | 816 (85.6) | 965 (80.2) | |
| Missing | 132 (11.9) | 204 (3.8) | 58 (6.1) | 46 (3.8) | |
| Ethnicity | |||||
| White | 809 (72.9) | 4247 (79.0) | 767 (80.5) | 963 (80) | 0.891 |
| Non-white | 164 (14.8) | 850 (15.8) | 133 (14) | 171 (14.2) | |
| Missing | 136 (12.3) | 279 (5.2) | 53 (5.6) | 70 (5.8) | |
| National Index of Multiple Deprivation quintile | |||||
| Least deprived | 284 (25.6) | 1515 (28.2) | 157 (16.5) | 367 (30.5) | 0.376 |
| Quintile 2 | 80 (7.2) | 535 (10.0) | 80 (8.4) | 116 (9.6) | |
| Quintile 3 | 126 (11.4) | 688 (12.8) | 115 (12.1) | 161 (13.4) | |
| Quintile 4 | 99 (8.9) | 635 (11.8) | 135 (14.2) | 137 (11.4) | |
| Most deprived | 157 (14.2) | 896 (16.7) | 192 (20.1) | 182 (15.1) | |
| Missing | 363 (32.7) | 1107 (20.6) | 274 (28.8) | 241 (20) | |