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Table 2 Summary of quantitative RMT measurement properties and findings

From: Development of the Myasthenia Gravis (MG) Symptoms PRO: a case study of a patient-centred outcome measure in rare disease

  Item # Tagetinga PSIb Disordered thresholdsc Item misfitd Dependencye
STEP 2: Draft scales
 FATIGUE-PRO: Physical fatigue 16 91% 0.94 0% 13% 7
 Muscle weakness 18 92% 0.90 33% 11% 15
 Muscle weakness fatigability 9 89% 0.86 22% 33% 3
STEP 2: Refined scales
 Physical fatigue 15 89% 0.95 0% 13% 6
 Ocular muscle weakness 3 + 2f 82% 0.57 0% 33% 0
 Bulbar muscle weakness 9 56% 0.81 10% 20% 2
 Muscle weakness fatigability Same as draft
 Respiratory muscle weakness N/A N/A N/A N/A N/A N/A
  1. N/A not applicable, Rasch measurement theory
  2. aPercentage of sample measurements covered by the scale: The higher percentage, the better targeting
  3. bPerson Separation Index (PSI) ranges from 0 to 1: Higher scores reflect higher reliability
  4. cPercentage of items displaying disordered response thresholds: Higher values suggest problems with the response scale
  5. dPercentage of items displaying statistical misfit on the basis of Chi-Square values summarising the difference between observed and expected scores, where Chi Square significance is estimated with a Bonferroni adjustment at p < 0.01
  6. eNumber of item pairs displaying item dependency in the form of residual correlations > 0.30
  7. fTwo new items generated within Step 2; quantitative data are not yet available for these items