Statistical analyses
disability, NDI, pain
Demographic and cross-sectional descriptive data was analyzed using parametric or non-parametric tests depending on the measure and normality of data.Differences between surgical groups and across three time periods (baseline, 10- and 20-year follow-ups) was analyzed using a 3 × 2 mixed design analysis of variance (ANOVA). Forty-five (VAS) and 42 (NDI) individuals completed ratings at all three time-points and were included in the ANOVA. Greenhouse-Geisser correction was used when the sphericity assumption was not met. Additional pairwise comparisons between time-points were performed if main effects were significant. Differences in improvements from baseline in VAS neck pain and NDI based on fusion status were analyzed with one-way analysis of covariance (ANCOVA), adjusted for baseline values.A p-value of < .05 was considered statistically significant. Effect sizes were considered small if Cohen’s d was > 0.2, intermediate if > 0.5, and large if > 0.8. Global outcome ratings of 1 (completely recovered) to 3 (better) were categorized as improved, whereas 4 (unchanged) to 6 (much worse) was categorizes as not improved. A 30 mm reduction in VAS and a change score of 20% units in the NDI were used as cut-offs to determine clinically relevant improvements (CRI) in pain intensity [Spearman’s rho was used to evaluate the relationship between pain and disability, and psychosocial factors.