Distribution of baseline (phase I) covariates by back pain in phases I–III (n=8865)
| Covariate (n missing) | Number of phases with reported back pain* | ||
| 0 | 1 | ≥2 | |
| Sex (0) | N (%) | ||
| Male | 3604 (61) | 1447 (24) | 891 (15) |
| Female | 1283 (47) | 782 (29) | 658 (24) |
| Age (0), years | |||
| 35–39 | 1381 (59) | 585 (25) | 366 (16) |
| 40–49 | 2267 (56) | 1035 (26) | 743 (18) |
| ≥50 | 1239 (54) | 609 (27) | 440 (19) |
| Occupational grade (0) | |||
| High (administrative) | 1642 (62) | 633 (24) | 374 (14) |
| Middle (professionals/executive) | 2404 (57) | 1087 (26) | 749 (18) |
| Low (clerical/support) | 841 (47) | 509 (29) | 426 (24) |
| Parents’ education (914) | |||
| High | 960 (61) | 356 (23) | 262 (17) |
| Intermediate | 864 (56) | 438 (28) | 250 (16) |
| Low | 2560 (55) | 1204 (26) | 857 (19) |
| Body mass index (11) | |||
| Recommended weight | 3068 (58) | 1332 (25) | 915 (17) |
| Overweight | 1533 (55) | 731 (26) | 526 (19) |
| Obese | 281 (51) | 162 (30) | 106 (19) |
| Job control (72) | |||
| High | 4015 (57) | 1787 (26) | 1192 (17) |
| Low | 1402 (53) | 696 (26) | 536 (20) |
| Job demands (44) | |||
| Low | 4084 (57) | 1812 (25) | 1270 (18) |
| High | 783 (54) | 404 (28) | 268 (18) |
↵*P values (χ2): cross-tabulations between covariates and number of pain reports were all significant (p values<0.05) except borderline for job demands (0.07).