**2.3 Statistical analysis**

Descriptive statistics were calculated for the study variables; continuous variables were summarized as mean (standard deviation, SD) and categorical ones as frequencies and percentages. Similarly, descriptive statistics were estimated for NAQ-22 items and overall score.

At the univariate analysis, the associations between NAQ-22 overall score with sociodemographic and work-related factors were evaluated with t-test and analysis of variance (ANOVA), as appropriate; the assumption of normality was verified with Shapiro-Wilk test.

At the multivariate approach, the independent associations between NAQ-22 score (treated as the dependent variable), sociodemographic and work-related factors (treated as independent variables) were evaluated by multivariate linear regression analysis. Model fit was evaluated with the examination of jackknifed residuals. The level of statistical significance was set at 0.05. Statistical analysis was performed with SPSS® version 16 statistical software (SPSS Inc, Chicago, IL, USA).
