The Oswestry Disability Index (ODI) was the most often used patient-reported outcome measure for low back pain. Incomplete ODI questionnaires can lead to measurement errors, leading to misclassification of impairment degree. With increasing numbers of missing items, researchers determined which scoring method—alternative scoring (AS) or multiple imputations (MI)—produced the best estimate of measurement error; and researchers analyzed the precision of AS and MI when scoring the ODI classifying disability level. Researchers conducted quantitative research. Researchers looked at 995 patients who had lumbar spine surgery and completed ODI data between 2014 and 2020. (1) By randomly setting items to missing, researchers simulated 1000 datasets for each number of ODI items (1–9) for which data could be missing; (2) measured simulated scores using AS or MI; (3) determined by calculating absolute percentage error (APE) of simulated vs true scores; (4) categorized disability level using the simulated score; and (5) compared APE between scoring methods to evaluate precision and misclassification rates. APE was 0.6%, 3.1%, and 12% for 1, 5, and 9 missing items using AS, respectively, and misclassification rates were 0.6%, 4.6%, and 13%. APE was 1.3%, 7.9%, and 56% for 1, 5, and 9 missing items using MI, respectively, and misclassification rates were 4.6%, 9.7%, and 58%. MI introduced a higher measurement error than When as the number of missing ODI elements increased. When evaluating ODI scores with incomplete data and defining impairment levels, clinicians should use APE. Assessments and treatment plans could be harmed by inaccurate interpretations.