Among substance users, discrete factor analysis for categorical data was conducted using Mplus (v. 6; Muthén and Muthén, 2010) to describe factor loadings of dependence symptoms. The Tucker-Lewis index (TLI), comparative fit index (CFI), standardized root mean square residual (SRMR), and the root mean square error of approximation (RMSEA) were used to assess the model fit for a one-factor model, as substance dependence is generally considered a single factor (Edwards and Gross, 1976; Wu et al., 2009a; Wu et al., 2009b). A higher value of TLI and CFI (>0.94) and a lower value of SRMR (<0.8) and RMSEA (<0.07) indicate an excellent fit to the data (Browne and Cudeck, 1993; Hu and Bentler, 1999). The one-factor model for substance dependence also was assessed by the scree test (Cattell, 1966) and the ratio of the first to the second eigenvalues.
