Maintaining instrument quality while reducing items: application of Rasch analysis to a self-report of visual...

Area of Science:

• Health Outcomes Research
• Psychometrics
• Ophthalmology

Background:

• Efficiency in health care outcome measurement is crucial.
• Developing shortened instruments with strong psychometric properties is often overlooked.
• Existing instruments may have limitations such as rating scale issues, ceiling effects, and item redundancy.

Purpose of the Study:

• To demonstrate the utility of Rasch analysis in reducing instrument items while preserving psychometric integrity.
• To refine the Visual Function-14 (VF-14) instrument for measuring vision-dependent activities related to cataract surgery.

Main Methods:

• Rasch analysis (BIGSTEPS) was applied to the original 14-item Visual Function (VF-14) instrument.
• The rating scale was converted to a three-point scale, and 10 additional items were added, creating a 24-item instrument (VF-24).
• Item reduction was performed based on Rasch analysis criteria (mean square, calibration levels) to create a 10-item instrument (VF-10).

Main Results:

• Initial Rasch analysis of the VF-14 revealed limitations including rating scale misuse, ceiling effects, and item redundancy.
• Modifications to the VF-24 improved person separation (2.55 to 2.99) and Cronbach's alpha (.83 to .91) but increased item redundancy.
• The final VF-10 demonstrated reduced item redundancy while maintaining comparable person separation (2.20) and Cronbach's alpha (.89) to the original VF-14.

Conclusions:

• Rasch analysis is effective for evaluating and refining health outcome instruments.
• The developed VF-10 is a more efficient and psychometrically sound instrument for assessing vision function after cataract surgery.
• This study highlights Rasch analysis as a valuable tool for creating shortened, valid, and reliable measurement instruments.