| A valid QoL measure refers to the extent to which a concept is well founded and corresponds accurately to the |
| “real world.” The validity of a QoL measurement is considered to be the degree to which the tool measures what it |
| claims to measure. |
| Three main properties must be explored: reliability, internal validity, and external validity. |
| Reliability |
| The reliability or internal consistency is the extent to which a measurement gives consistent results, that is, the extent with |
| which a set of items in a dimension measures the same attribute. Reliability is assessed by the computation of |
| Cronbach’s alpha coefficients. Cronbach’s alpha coefficients higher than 0.70 result in satisfactory reliability. |
| Internal Validity |
| Two main aspects must be considered: content validity and construct validity. |
| (i) Content validity is a nonstatistical type of validity that involves the examination of the questionnaire content to |
| determine whether it covers all the aspects of the domain to be measured. |
| (ii) Construct validity refers to the extent to which the questionnaires developed from a theory do actually measure what |
| the theory says they do. It mainly relies on statistical analyses of the internal structure of the questionnaire including |
| the relationships between responses to different items. Construct validity was assessed by performing the following. |
| (A) Exploratory or confirmatory factorial analyses: in the case of confirmatory factorial analysis, a Kaiser-Meyer-Olkin |
| (KMO) measure higher than 0.50 and a total variance higher than 70% indicate that the number of identified factors |
| (or QoL dimensions) fit to the model. |
| (B) Rash analysis to explore the unidimensionality of each domain identified: unidimensionality is retained if item |
| goodness-of-fit (INFIT) statistics values range from 0.7 to 1.2. |
| (C) Computation of correlation coefficients: correlation coefficients of each item with its dimension (item internal |
| consistency (IIC)) higher than 0.40 and higher than the correlation coefficients of this item with other dimensions (item |
| discriminant validity (IDV)) reflect a satisfactory construct validity. |
| External validity |
| External validity concerns the extent to which the internal construct can be support by external criteria. External |
| validity relies on assessment of the following. |
| (i) Convergent validity: relationships between the dimensions of the questionnaire and the dimensions of other previously |
| validated questionnaires measuring the same concept. |
| (ii) Criterion validity: relationships between the dimensions of the questionnaire and other features: sociodemographic |
| or clinical features. |
