BACKGROUND: The GAIN Substance Problem Scale (SPS) measures alcohol and drug problem severity within a DSM-IV-TR framework. This study builds on prior psychometric evaluation of the SPS by using Rasch analysis to assess scale unidimensionality, item severity, and differential item functioning (DIF). METHODS: Participants were attending residential or outpatient treatment in Alberta and Ontario, Canada, respectively (n=372). Rasch analyses modeled a latent problem severity continuum using SPS scores at treatment admission and 6-week follow-up. We examined DIF by gender, treatment modality (outpatient vs. residential), and assessment timing (baseline vs. follow-up). RESULTS: Model fit was good overall, supporting unidimensionality and a single underlying continuum of substance problem severity. Relative to person severity, however, the range of item severities was narrow. Items were too severe for many clients to endorse, particularly at follow-up. Overall, the rank order of item severities was stable across gender, treatment modality, and time point. Although traditional Rasch criteria indicated a number of statistically significant and substantive DIF estimates across modality and time points, effect size indices did not suggest a net effect on total scale scores. CONCLUSIONS: The analysis broadly supports use of the SPS as an additive measure of global substance severity in men and women and both residential and outpatient settings. Although DIF was not a major concern, there was evidence of item redundancy and suboptimal matching between items and persons. Findings highlight potential opportunities for further improving this scale in future research and clinical applications of the GAIN.
BACKGROUND: The GAIN Substance Problem Scale (SPS) measures alcohol and drug problem severity within a DSM-IV-TR framework. This study builds on prior psychometric evaluation of the SPS by using Rasch analysis to assess scale unidimensionality, item severity, and differential item functioning (DIF). METHODS:Participants were attending residential or outpatient treatment in Alberta and Ontario, Canada, respectively (n=372). Rasch analyses modeled a latent problem severity continuum using SPS scores at treatment admission and 6-week follow-up. We examined DIF by gender, treatment modality (outpatient vs. residential), and assessment timing (baseline vs. follow-up). RESULTS: Model fit was good overall, supporting unidimensionality and a single underlying continuum of substance problem severity. Relative to person severity, however, the range of item severities was narrow. Items were too severe for many clients to endorse, particularly at follow-up. Overall, the rank order of item severities was stable across gender, treatment modality, and time point. Although traditional Rasch criteria indicated a number of statistically significant and substantive DIF estimates across modality and time points, effect size indices did not suggest a net effect on total scale scores. CONCLUSIONS: The analysis broadly supports use of the SPS as an additive measure of global substance severity in men and women and both residential and outpatient settings. Although DIF was not a major concern, there was evidence of item redundancy and suboptimal matching between items and persons. Findings highlight potential opportunities for further improving this scale in future research and clinical applications of the GAIN.