Table 4 QoL terminology [38, 41]

Reliability

Extent to which the questionnaire is repeatable and consistently produces the same results

Validity

Degree to which the questionnaire measures what it is intended to measure

Internal consistency

How well the items of a questionnaire relate to each other and to the total questionnaire. It is most commonly evaluated by Cronbach’s alpha. An alpha ≥0.70 indicates good internal consistency

Internal validity

Internal structure of the questionnaires and is usually evaluated by factor analysis, inter-items correlations and floor and ceiling effects

Test–retest

Reproducibility of the questionnaire over time. The questionnaire is completed on two occasions by the same patients in whom no change in the condition has taken place. It is most commonly evaluated by the intraclass correlation coefficient (ICC). An ICC ≥0.70 indicates good test–retest reliability

External validity

Relationship between the questionnaire and an external criterion (e.g. other measures of the same or different dimensions of health), and the most common types are face, content, convergent/discriminant and construct validity

Face validity

Determined by expert opinion as to whether the questionnaire seems to measure HRQL related to the disease in question. Least rigurous form of validity. Type of external validity

Content validity

Based on subjective assessment of the extent to which a questionnaire represents all dimensions of a construct. Type of external validity

Convergent/discriminant validity

Assessed by calculating the correlation between the questionnaire and measures of similar or dissimilar constructs. Type of external validity

Construct validity

Ascertained by calculating the correlation between the questionnaire and an independent measure, which reflects the severity of the disease in question. Type of external validity