Face:

This is judged by the person using the scale.

The user studies the scale then provides a subjective judgment regarding its
apparent value based on criteria such as length, item labels, modes of response etc.

This is a simple superficial judgement based on the user's opinion.

It is not because a tool appears valid that it actually is!

Content:

A qualitative phase which should be carried out at the start of the development of a scale. It consists of determining the items which make up the scale through a survey of experts. Patients are now considered as experts.

Criterion:

The construct is evaluated by both the scale which is being tested and by a reference criterion (Gold Standard). A sufficient number of subjects are evaluated independently by the two tools and then the strength of the statistical relationship between the two is evaluated using a correlation coefficient.

The validity is said to be concomitant when the evaluations are carried out simultaneously.

Validity is said to be predictive when the construct is first measured by the scale, then by a reference criterion several weeks or months later.

Construct:

The aim is to reinforce the validity of a scale which measures a particular construct by correlating the scale to successive theoretical hypotheses.

Convergent validity: refers to the degree to which the scores of the tested scale are correlated with other variables in the same domain.

Divergent validity: refers to a low correlation between the scores of the scale and theoretical variables from a different evaluation field.

Discriminant validity: refers to the capacity of the scale to obtain significantly different scores between two populations with different characteristics.

Intra-rater:

The intra-rater reliability of a test relates to the stability of the scores obtained by a rater when he/she carries out the test on two separate occasions. A single rater tests each patient twice (or more) with several days in between each test. The patient's state must remain unchanged during this time.

1) Quantitative measure:
- Intraclass correlation coefficients (ICC)
- Bland and Altman method (fidelity between two raters)

2) Qualitative measure: Two coefficients, Kappa or weighted Kappa

Inter-rater:

The inter-rater reliability of a test describes the stability of the scores obtained when two different raters carry out the same test. Each patient is tested independently at the same moment in time by two (or more) raters.

1) Quantitative measure:
- Intraclass correlation coefficients (ICC)
- Bland and Altman method (fidelity between two raters)

2) Qualitative measure: Two coefficients, Kappa or weighted Kappa

Test-retest:

The test-retest reliability of a test describes the stability of scores obtained by a patient when he/she is evaluated on two separate occasions. This appears similar to intra-rater reliability but in this case, the patient self-evaluates him/herself (for example a pain-rating scale).

1) Quantitative measure:
- Intraclass correlation coefficients (ICC)
- Bland and Altman method (fidelity between two raters)

2) Qualitative measure: Two coefficients, Kappa or weighted Kappa

Internal consistency:

Cronbach's Alpha coefficient is used to measure the internal consistency of a group of items relating to a single clinical domain: in other words, how the items correlate with each other.

The internal consistency is tested during the development of the scale.

Responsiveness :

A tool is said to be sensitive to change if it can precisely measure increases and decreases in the construct measured.

This is important for tools which are used to evaluate changes following a therapeutic action. The aim is to measure the capacity of the scale to detect small but clinically significant changes.

When an outcome measure is sensitive to change, the score increases as the patient improves, decreases as the patient worsens and does not change if the patient's state remains stable.