Equating the MOS SF36 and the LSU HSI Physical Functioning Scales

Area of Science:

• Health Measurement
• Psychometrics
• Biostatistics

Background:

• The Medical Outcomes Study Short Form 36 (SF36) and Louisiana State University Health Status Instruments (LSU HSI) are used to assess physical functioning.
• Equating these instruments allows for standardized comparison of physical functioning scores across different patient populations and studies.

Purpose of the Study:

• To equate the physical functioning subscales of the SF36 and LSU HSI.
• To establish a common unit of measurement for physical functioning scores derived from both instruments.
• To provide conversion tables for transforming scores between the two scales.

Main Methods:

• Utilized Rasch rating scale models (separate and mixed) to analyze data from 285 patients.
• Fit the SF36's 10-item physical functioning scale (PF10) and the LSU HSI's 29-item Physical Functioning Scale (PFS).
• Correlated difficulty estimates of similar items to assess measurement convergence.

Main Results:

• Item difficulty estimates from both instruments were highly correlated (.95), indicating they measure the same construct.
• Equated measures showed a strong correlation (.80, .86 disattenuated).
• The LSU HSI (PFS) demonstrated lower error, better model fit, and higher reliability coefficients compared to the SF36 (PF10).

Conclusions:

• The SF36 and LSU HSI physical functioning scales can be equated and used to measure physical functioning in a common metric.
• The LSU HSI appears to be a more precise and reliable measure of physical functioning.
• Conversion tables facilitate the integration of data from both instruments, enhancing comparability in health status assessments.