Overall levels of evidence for the quality of the measurement property.26 27 The quality of the evidence for the measurement property for each PROM, considering the quality criteria for each measurement property (online supplementary file 2), the methodology of each study reporting the measurement property (table 4) and the number of studies reporting the measurement property including consistency of findings
| Level of evidence | Rating | Quality criteria |
| Strong | +++ or − − − | Consistent findings (positive or negative) in multiple studies of good methodological quality OR in one study of excellent methodological quality |
| Moderate | ++ or − − | Consistent findings (positive or negative) in multiple studies of fair methodological quality OR in one study of good methodological quality |
| Limited | + or − | One study of fair methodological quality (positive or negative) |
| Conflicting | +/− | Conflicting results |
| Unknown | ? | Only studies of poor methodological quality |
+=positive rating, ?=indeterminate rating, −=negative rating.
PROM, patient-reported outcome measure.