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. |