This study relied on a cross-sectional design and was performed in the neurology department of a public French academic teaching hospital (Marseille, France). The inclusion criteria were as follows: a MS diagnosis according to the McDonald criteria , age ≥ 18 years, any subtype of MS, outpatient status, no neurological disease (other than MS), no history of severe mental illness (except depressive disorder), no dementia (Mini Mental State Examination score < 24), no history of alcohol/drug abuse, and native French-speaking. The French Ethics Committee approved the study (Comité Consultatif de Protection des Personnes dans la Recherche Biomédicale, Marseille 2, France) and patients gave their written consent to participate. Sociodemographic (age, gender, marital status, education level, and occupational activity) and clinical (MS subtype and disease duration) data were recorded for each patient. MS disability was assessed using the Expanded Disability Status Scale (EDSS).
QoL was assessed using the French version of the Multiple Sclerosis International Quality of Life questionnaire (MusiQoL) and the Short Form 36 (SF36). The MusiQoL is a well-validated MS-specific questionnaire that describes the nine dimensions and yields a global index score: activity of daily living (ADL), psychological well-being (PWB), symptoms (SPT), relationships with friends (RFr), relationships with family (RFa), relationships with health care system (RHCS), sentimental and sexual life (SSL), coping (COP), and rejection (REJ). The SF36 is a generic questionnaire  describing eight subscales: physical function (PF), social function (SF), role physical (RP), role emotional (RE), mental health (MH), vitality (V), bodily pain (BP), and general health (GH). Two composite scores (physical and mental, PCS-SF36 and MCS-SF36) were also calculated.
Attention performance was measured using two tests: the French version of the Paced Auditory Serial Addition Test (PASAT)  and the attention/concentration subscale of the French version of the Wechsler Memory Scale (attention_WMS) . The attention_WMS score was calculated using 3 subtests (mental control, digit span, and visual memory). By comparing with French normative values according to age group , a patient was considered cognitively impaired if their score was more than 1 standard deviation (SD) below the normative value. The PASAT score was generated using the PASAT-3 s version of the test (number of correct responses). By comparing with French normative values according to sex, age, and educational level , a patient was considered to be cognitively impaired if their PASAT score was more than 1 SD below the normative value.
Statistical analyses were performed on the populations defined above using the same procedure reported in the initial validation publication (reference population) . For each group, psychometric properties were compared to those reported from the reference population. The structures of the MusiQoL were explored in the non-impaired and impaired populations using principal component factor analyses with varimax rotation [19, 20] to determine how these structures matched with the initial structure of the MusiQoL. For each population, the proportion of factors identified from the initial nine-factor structure of MusiQoL and the proportion of items projected to their initial dimension were retrieved.
The multidimensional structure (construct validity) was verified using the multi-trait/multi-item analysis program . Internal structural validity was assessed by calculating item-dimension correlations. Item internal consistency (IIC) was calculated by correlating each item with its scale, and item discriminant validity (IDV) was assessed by determining the extent to which items correlated with the dimension they were hypothesized to represent compared to correlations with other dimensions. Floor and ceiling effects were determined to assess the distribution of the responses. For each dimension, internal consistency reliability was evaluated using Cronbach’s alpha coefficient .
The unidimensionality of each dimension was calculated by computing item goodness-of-fit statistics (INFIT) issued from Rasch analyses . INFIT values ranging from 0.7 to 1.2 ensure that all scale items tend to measure the same concept.
To assess external validity, Spearman’s correlation coefficients were used to determine relationships between the MusiQoL and SF36 dimensions in each group, and the associations between MusiQoL dimension scores and sociodemographic and clinical features were reported as in the validation study. For qualitative variables, the mean dimension scores of the MusiQoL were compared across patient groups (e.g., gender, educational level, marital status, and occupational status) using one-way analysis of variance. Quantitative variables (e.g., age, EDSS score, and MS duration) were analyzed using Spearman’s correlation coefficients. Acceptability was assessed by calculating the percentage of missing data per dimension. Data analyses were performed using SPSS 11.0, MAP-R, LISREL and WINSTEP software.
To quantify how each of the 4 structures matched with the initial structure (reference structure), suitability indices were calculated in accordance with a previous study . Decision rules were established by experts in QoL and used to define satisfactory properties according to appropriate standards [19, 20]. The means of different proportions were calculated to produce the suitability index of the ‘construct validity’ and the suitability index of the ‘external validity’.