Table 2.

Levels of evidence for the quality of the measurement properties for PROMs, taken from Terwee et al. 14

Level	Rating*	Criteria
Strong	+++ or −−−	Consistent findings in multiple studies of good methodological quality OR in one study of excellent methodological quality
Moderate	++ or −−	Consistent findings in multiple studies of fair methodological quality OR in one study of good methodological quality
Limited	+ or −	One study of fair methodological quality
Conflicting	+/−	Conflicting findings
Unknown	?	Only studies of poor methodological quality

*Positive rating, ‘+’; Indeterminate rating, ‘?’; Negative rating, ‘−’.