Refinement and reduction of the Impact of Psoriasis Questionnaire: classical test theory vs. Rasch analysis

Area of Science:

• Dermatology
• Psychometrics
• Health Outcomes Research

Background:

• Quality of life instruments are crucial for assessing disease severity.
• Existing instruments may lack standardization and require refinement.
• Psychometric analyses offer methods to improve and shorten these tools.

Purpose of the Study:

• To psychometrically test and refine the Impact of Psoriasis Questionnaire (IPSO).
• To compare classical test theory (CTT) and Rasch analysis for instrument optimization.
• To develop a more efficient and reliable measure for psoriasis patients.

Main Methods:

• Utilized classical test theory (CTT) and Rasch analysis on data from 792 psoriasis patients.
• Tested and optimized the 16-item Impact of Psoriasis Questionnaire (IPSO).
• Compared two shortened versions derived from CTT and Rasch models.

Main Results:

• CTT analysis revealed suboptimal items and inappropriate subscales in the original IPSO.
• Rasch analysis resulted in a unidimensional 11-item questionnaire with improved psychometric properties.
• The Rasch-reduced version demonstrated better reliability and unidimensionality compared to the CTT version.

Conclusions:

• The Impact of Psoriasis Questionnaire (IPSO) can be effectively improved and shortened.
• The Rasch-reduced IPSO is a preferred, short, reliable, and unidimensional instrument for assessing the psychosocial impact of psoriasis.
• This refined instrument enhances the evaluation of patient quality of life in moderate to severe psoriasis.