A strength of the present study is that only validated and
common instruments were used. Consistent use of outcome
measures recommended by the MMCIS Statement will
facilitate comparison of results with other studies. A further
strength is that baseline characteristics of participants and
individuals lost to follow up did not show significant differences,
except for a lower mental health, and higher
depression and affective pain scores for those individuals
lost to follow up. This is typical for study populations
where the healthier individuals stay in the study. Without
this bias, the predictive value of the psychological factors
would have been even higher because of variance restriction
in the criterion.