Table 1.

Quality assessment criteria

1	Did the sample represent the population of interest?
2	Was loss to follow-up <35% and response rate >65%?
3	Were the prognostic factors measured with valid and reliable instruments?
4	Was the outcome of the study objectively measured?
5	Were important potential confounders (ie, age, gender, prior WA and comorbidity) appropriately accounted for in the analysis?
6	Was the statistical analysis appropriate?

WA, work absence.