Individual standardized factor loadings and modification indices (MI) for each of the original items were evaluated for item reduction, supported on theoretical considerations regarding items' content, CVR and factor manifestations.
To prepare data for principal component analysis and item reduction, items were checked for inter-item correlation, as this would suggest redundancy.
Items were eliminated during the reduction phase with missing values exceeding 5%, high inter-item correlation (r >0.80), floor or ceiling effects over 70%, or an absolute value of skewness exceeding 4.0.
For continuous variables mean (SD) and range are presented ALS, amyotrophic lateral sclerosis; COPD, chronic obstructive pulmonary disorder; HF, heart failure; MS, multiple sclerosis To ensure a stable factor solution, further reductions of items were needed before doing the PCA.
In addition, when items were removed during the item reduction process, the factor structure did not withstand these changes, indicating that the factor structure was not stable.
Individual quality assessment items were also analysed for risk reduction, including the use of mortality as primary end-point in the studies, the use of adequate blinding, and the presence of crossover phenomena.
In the first reduction step, items were reduced by MS and BM to 48 (Table 3) based on six criteria: similar items, not relevant enough for MCD, too academic, too vague, too unspecific to be categorised to only one domain, too abstract or difficult to measure (Table 5).
Through the empirical item reduction process, 13 items were selected from the 37-item pool generated during qualitative development.
Analysis of the results allowed a further reduction where the items were listed according to clarity.
Of the 1,400 items in the database, 624 were classified into these five domains and these items were considered for further testing and reduction.
