Psychometric criteria
| Psychometric property | Criteria | |
|---|---|---|
| EDSS, Expanded Disability Status Scale; GNDS, Guy’s Neurological Disability Scale; MS, multiple sclerosis; MSIS-29, Multiple Sclerosis Impact Scale; PP, primary progressive; RR, relapsing–remitting; SP, secondary progressive. | ||
| Data quality | Missing items <10% | |
| High percentage of computable scale scores | ||
| High item test–retest reliability: r ⩾0.50 | ||
| Scaling assumptions | Similar response option frequency distribution | |
| Skewness between −1 and +1 | ||
| Similar means and standard deviations | ||
| Similar and substantial item-total correlations: r >0.30 | ||
| Acceptability | Scores should span the full scale range | |
| Mean scores should be around the midpoint | ||
| Floor and ceiling effect <20% | ||
| Skewness between −1 and +1 | ||
| Reliability | Cronbach’s α >0.80 | |
| Scale test–retest reliability (ICC) >0.80 | ||
| Responsiveness | Correlation between change score and transition score should be high | |
| Clinically important difference: 0.5 = small, 1.0 = moderate, 1.5 large | ||
| Validity | ||
| Internal validity | Moderate intercorrelation: r = 0.30–0.70 | |
| External validity | High correlation between MSIS-29 physical scale and EDSS | |
| Low correlation between MSIS-29 psychological scale and EDSS | ||
| High correlation between MSIS-29 physical scale and GNDS | ||
| Low correlation between MSIS-29 psychological scale and GNDS | ||
| Group differences validity | Similar and low correlations between men and women | |
| Similar and low correlations for age | ||
| Hypotheses | Mean physical MSIS-29 score for different groups of the EDSS score: | |
| MSIS-29 score (EDSS 0.0–3.5) < MSIS-29 score (EDSS 4.0–6.0) | ||
| MSIS-29 score (EDSS 0.0–3.5) < MSIS-29 score (EDSS ⩾6.5) | ||
| MSIS-29 score (EDSS 4.0–6.0) < MSIS-29 score (EDSS ⩾6.5) | ||
| Mean physical MSIS-29 score for different groups of the GNDS score: | ||
| MSIS-29 score (GNDS <15) < MSIS-29 score (GNDS ⩾15) | ||
| Mean physical and psychological MSIS-29 scores for different MS subtypes: | ||
| MSIS-29 score (RR) < MSIS-29 score (SP) | ||
| MSIS-29 score (RR) < MSIS-29 score (PP) | ||
| MSIS-29 score (SP) < MSIS-29 score (PP) | ||