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.
Box 1: Definitions of the main psychometric properties of a QoL measure.