The MRC data of each muscle group were treated as if it was an ‘item’ that needed to be completed by the patients with response options from 0 to 5 (in the current study setting: a physician completed the ‘item’) using the Rasch Unidimensional Measurement Model 2020 software (Andrich et al., 2003 ).
In Analysis 1 (MRC Rasch in each cohort separately), the following person factors were taken into account (Supplementary Table 2):

Ages: 1, <40 years; 2, 40–59 years and 3, ≥60 years;

gender: 0, female; 1, male;

type of disease: (a) inflammatory neuropathy-cause-and-treatment cohort: 1, Guillain–Barré syndrome; 2, CIDP; 3, gammopathy related polyneuropathy; (b) myotonic dystrophy cohort: 1, mild; 2, adult; 3, child/congenital type; and (c) limb-girdle dystrophy cohort: 1, sarcoglycanopathy; 2, calpainopathy; 3, limb-girdle type 1B, 2B and 2I; 4, unclassified;

duration of disease: (a) for all cohorts except limb-girdle patients: 1, <5 years; 2, 5–9 years; 3, 10–19 years; 4, ≥20 years; and (b) for limb-girdle cohort: 1, <10 years; 2, 10–19 years; 3, 20–29 years; 4, ≥30 years;

physician's experience in the neuromuscular field: for the inflammatory–neuropathy cause and treatment studies: 1, <3 years experience; 2, 3–5 years experience; 3, ≥6 years experience; the latter group constituting senior neuromuscular experts;

institution; for the Guillain–Barré syndrome trials: 0, community based; 1, university based hospital; and

country; for the Guillain–Barré syndrome cohort 2004: 1, The Netherlands; 2, Germany; 3, Belgium.

For Analyses 2 and 3 (MRC Rasch after pooling data and MRC sum score in Guillain–Barré syndrome/CIDP), the factors studied included (i) age category: 1, <40 years; 2, 40–59 years; 3, ≥60 years; (ii) gender: 0, female; 1, male; and (iii) type of disease: depending on the amount of illnesses being pooled together, each illness received a separate code.
For the MRC sum score analysis, the person separation index was also determined, which should be ≥0.7 for proper group comparison, and a minimum of 0.9 for clinical use (Bland and Altman, 1997 (link)). The unrestricted partial credit Rasch model was used. Further analyses were undertaken using Stata 11.0 statistical software for Windows XP.