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

Table 2 Summary of quantitative RMT measurement properties and findings

	Item #	Tageting^a	PSI^b	Disordered thresholds^c	Item misfit^d	Dependency^e
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 + 2^f	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

N/A not applicable, Rasch measurement theory
^aPercentage of sample measurements covered by the scale: The higher percentage, the better targeting
^bPerson Separation Index (PSI) ranges from 0 to 1: Higher scores reflect higher reliability
^cPercentage of items displaying disordered response thresholds: Higher values suggest problems with the response scale
^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
^eNumber of item pairs displaying item dependency in the form of residual correlations > 0.30
^fTwo new items generated within Step 2; quantitative data are not yet available for these items

ISSN: 1750-1172