GAIN Working Papers: Rasch Analysis of the Internal Mental Distress Scale

Purpose. The purpose of this report is to provide a brief psychometric analysis of the Internal Mental Distress Scale using the Rasch measurement model. The 43-item IMDS is a count of past-year symptoms related to internalizing disorders, including somatic, anxiety, depression, traumatic stress and suicide thoughts. It is based on the DSM-IV-TR, the Hopkins Symptom Checklist 90 (HSCL-90), the Mississippi Scale of Post Traumatic Stress Disorder (PTSD) and common screening items for suicide risk (homicidal thoughts, suicidal thoughts, plans, means, attempts). Methods. Data were analyzed on 7,435 persons who presented for substance problem screening. Rasch analysis included an examination of: person and item reliabilities; construct validity including item and person fit statistics; and differential item functioning (DIF) across subgroups. DIF analysis allowed us to determine if the relative item estimates (i.e., item difficulty estimates) remained invariant across subgroups of persons. Results. The IMDS performs well as a measure of the construct of internal mental distress. Both items and scales form the theoretically expected hierarchies with a person internal consistency reliability of .89 and an item reliability of 1.00. The persons' responses generally conformed to the expectations of the Rasch model. Of the 43 items in the IMDS, significant DIF (i.e., > .5 SD = .58 logits) occurred in 3 items for males vs. females, 11 items for youth vs. adults, 4 items for race when using African American as the reference group, and 17 items for primary substances when using alcohol as the referent. No items misfit using the criterion of .75-1.33 MNSQ for both infit and outfit. Conclusion. The IMDS functioned well as a unidimensional measure with good person and item reliability. In terms of item quality, there were no items with both infit and outfit values outside of our criterion of .75-1.33 logits. While there were several items with high outfit values, these indicated that a few people endorsed these items unexpectedly, but that, otherwise, the items performed well as indicators of the general construct of internal mental distress. While the items concerning suicidal ideation fit well in terms of both infit and outfit, they were the chief reason for persons to have high outfit values. These high person outfit values revealed persons with atypical suicidal ideation. In other words, these were people with suicidal ideation that did not endorse the usual less severe symptoms that typically precede or accompany suicidal ideation. Differential item functioning was not a major issue in terms of causing biased IMDS measures, but the DIF analyses did reveal some interesting group differences that may merit further study. This is especially true of the primary drug severity group. Citation. Conrad, K. J., Conrad, K. M., Dennis, M. L., & Riley, B.B. (2008). GAIN working papers: Rasch analysis of the Internal Mental Distress Scale. Chicago, IL: Chestnut Health Systems. Retrieved from http://www.chestnut.org/li/gain/psychometric_reports/gain_working_papers_rasch_analysis_of_the_internal_me ntal_distress_scale.pdf