Abstract
BACKGROUND: The purpose of this study is to use Rasch analysis to explore the validity of considering self-report scores from Wisconsin Upper Respiratory Symptom Survey (WURSS-21) as a single global illness severity domain. The WURSS-21 is a widely used questionnaire instrument that assesses symptom severity and functional impact of common cold and flu-like illness. METHODS: This study applies item response theory, specifically Rasch modeling, to investigate dimensional and measurement properties of the WURSS-21, and looks at invariance over time. The data assessed represents 1167 people, each scoring the WURSS-21 once daily for up to seven consecutive days of acute upper respiratory infection (URI) illness. RESULTS: Rasch analysis supports a single domain WURSS-21 global symptom score. Assessment of differential item functioning across seven days of illness provides evidence for measurement invariance. While individual items rating physical symptoms were somewhat variable, items rating functional impairment and quality of life impact appeared quite consistent across a single domain over seven days of illness. CONCLUSION: Rasch analysis of WURSS-21 items provides evidential support for a single invariant domain. These findings support the practice of using a simply summed daily global illness severity score to represent the overall symptomatic and functional impairments arising from URI.