Table 2. Summary of quality of both measures.

Statistics	Cat-PROM5	Catquest-9SF
Person reliability index	0.90	0.88
Cronbach’s α	0.89	0.92
Variance explained	72%	64%
Variance explained by patients	55%	48%
Variance explained by items	16%	16%
Variance unexplained	28%	36%
Highest residual eigenvalue	1.5	1.6
Number of items	5	9
Number of misfitting items (Infit/Outfit out of range 0.7–1.3)	0	2
Number of reversed thresholds	0	0
Number of reversed category means	0	0
Number of statistically significant DIF instances	3 (7.5%)	3 (4.2%)
Number of instances of ‘large’ DIF, i.e. |DIF|≥0.64 Logitsa	1 (2.5%)	2 (2.8%)
Cohen’s standardized effect sizeb	−1.45	−1.47
Cohen’s standardized effect sizec	−1.09	−1.14

^aEducational Testing Service criteria http://www.ets.org/Media/Research/pdf/RR-12-08.pdf).

^bDenominator as SD from preop time point.

^cDenominator as SD for the whole sample (including both pre- and postoperatively).