Study Design. Cross-sectional study. Objective. The aim of this study was to assess the structural validity of the Brazilian version of the Neck Disability Index (NDI) in patients with chronic neck pain. Summary of Background Data. NDI is widely used in clinical and scientific contexts, although its structure has not been evaluated in the Brazilian version. Methods. Native Brazilian Portuguese speakers, aged ≥18 years, with neck pain complaint of at least 3 months, and minimal pain rating of 3 points at rest or during neck movements were included. Exploratory factorial analysis (EFA) and confirmatory factorial analysis (CFA) were used. In EFA, the adequacy of the model was assessed using Bartlett test of sphericity and Kaiser-Meyer-Olkin test. In CFA, the goodness-of-fit was assessed by the indices: root mean square error of approximation with 90% of confidence interval, comparative fit index, Tucker-Lewis Index, standardized root mean square residual, and χ2/degree of freedom. Akaike information criterion (AIC) and Bayesian information criterion (BIC) were considered to compare the models. Results. Two hundred fifty-four participants with chronic neck pain were included. The NDI model with one domain and five items presented the most adequate goodness-of-fit indexes and the lowest values of AIC and BIC, when compared with models with one domain and 10, eight, or seven items, and with the model with two domains and 10 items. Conclusion. In the Brazilian context, the NDI version with one domain and five items (personal care, concentration, work, driving, and recreation) presents the best structure according to the factorial analysis. Level of Evidence: 5

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Objective To translate, cross-culturally adapt and assess measurement properties of the translated version of the Northwick Park Neck Pain Questionnaire into Brazilian Portuguese. Design Cross-sectional study. Settings University healthcare facility and online. Participants People with chronic neck pain (n = 178). Interventions Not applicable. Main outcome measures Participants answered the translated version of Northwick Park Neck Pain Questionnaire, the Neck Disability Index, the Numerical Rating Scale for pain, the Tampa Scale for Kinesiophobia and the Pain Catastrophizing Scale. Seven to ten days after that, a subset of 84 participants answered the Northwick Park Neck Pain Questionnaire again. The structural (exploratory and confirmatory factor analyses) and construct validities, internal consistency, reliability and concordance were assessed. Level of significance was set at 5%. Results Participants’ (35.6 ± 13.5 years old) with symptoms duration of 54.4 ± 60.4 months scored 25.5 ± 14.0 on the Brazilian Northwick Park Neck Pain Questionnaire and 11.9 ± 5.8 on the Neck Disability Index. The structural analysis showed that the short version of the Northwick Park Neck Pain Questionnaire has an adequate structure to measure disability due to neck pain. Correlations with other questionnaires were between 0.268 and 0.678, Cronbach's alfa was 0.76, intraclass correlation coefficient was 0.96, standard error of measurement was 2.74 and minimal detectable change was 7.60. Conclusion The short version of the Northwick Park Neck Pain Questionnaire is valid and reliable to be used in patients with chronic neck pain, as it presented adequate measurement properties of structural and construct validity, reliability and concordance.

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