| n | % |
---|---|---|
Total sample | 2,351 | 100.0 |
Participation survey 1 (1990–91) | 1,782 | 75.8 |
Non-participation survey 1 | 569 | 24.2 |
   deceased | 6 | 0.2 |
   ill | 36 | 1.5 |
   unable to attend | 48 | 2,0 |
   unwilling to attend | 157 | 6.7 |
   did not attend, unknown reason | 304 | 12.9 |
   not retrievable | 11 | 0.5 |
   moved | 7 | 0.3 |
Participation survey 2 | 1,589 | 67.6 |
Non-participation survey 2 | 762 | 32.4 |
   deceased | 143 | 6.1 |
   ill | 49 | 2.08 |
   unable attend | 82 | 3.49 |
   unwilling to attend | 192 | 8.17 |
   did not attend, unknown reason | 294 | 12.5 |
   not retrievable | 2 | 0.1 |
Participation in both surveys | 1,405 | 59.81 |
Specific back diagnosis survey 1 | 58 | 2.5 |
Study cohort for analyses | 1,347 | 57.3 |