Scale structure
The scale structure refers to the set formed by the different items of the questionnaire, representing a certain construction (if they evaluate QoL or severity), and can be tested, for example, by factor analysis. Factor analysis is most commonly used to test the uni-dimensionality of the construct and is based on correlations of the items. If the correlation factor for a particular item is <0.40, those items can be removed from the questionnaire, as they do not show to belong to the set of questions in the remaining items 5,7. Factor analysis can be comprised into: exploratory factor analysis and confirmatory factor analysis. At times, a Pearson’s correlation coefficients in order to explore all interactions of items pairs and to be excluded before conducting factor analyses 8.