Methodological considerations of measuring disability in bipolar disorder: validity of the Multidimensional Scale of Independent Functioning

Objective: Recent studies have highlighted the prevalence, severity and persistence of the disability associated with bipolar disorder (BPD). Reliable instruments are needed to support research into the factors associated with disability and treatment response. Contextual factors (e.g., availability of supported employment programs) can affect functionality, posing a challenge to such investigations. We present preliminary findings regarding the validity of the Multidimensional Scale of Independent Functioning (MSIF) in BPD. The MSIF provides discrete ratings of support separate from both role responsibility and performance quality in work, residential and educational environments. These distinctions allow the 'correction' for variability explained by contextual factors that allows the comparison of studies conducted in different environments and time. Methods: Participants with BPD were administered the MSIF, the Social Adjustment Scale II (SAS-II) and an interview recording objective data regarding work, school and residential activities as part of an ongoing longitudinal study of BPD disability. Results: Construct validity estimated using standardized Cronbach's alpha coefficient was 0.76 (n = 58). MSIF global ratings were significantly lower (reflecting higher functionality) for subjects engaged in productive activity compared with participants who were not active (t = -3.6, p = 0.001) demonstrating external validity. Inter-rater reliability estimates ranged from 0.86 to 0.99 (n = 49). Significant, high correlations were demonstrated between comparable MSIF and SAS-II global ratings (criterion validity = 0.70-0.79) and low correlations were found between non-comparable ratings (discriminant validity = -0.07 to -0.35) (n = 14). Conclusion: We conclude that the MSIF is a valid and reliable instrument optimally designed for studying determinants of disability and treatment response in BPD.