Table 2.

Psychometric properties of the Motivation to Use CPAP Scale at item level (n = 183)

	Factor loading*	h2**	Infit MnSq***	Outfit MnSq****	Difficulty	DIF contrast across gendera, b	DIF contrast across sleepiness conditiona, c	DIF contrast across insomnia conditiona, d
Factor 1 “Autonomous motivation”/items
1. I use the CPAP treatment because it makes me feel good.	0.721	0.554	1.10	1.07	− 0.28	0.18	− 0.06	0.13
2. I use the CPAP treatment because I want to avoid having apneas.	0.773	0.611	1.01	0.93	0.95	− 0.49	− 0.33	− 0.44
3. I use the CPAP treatment because I want to feel more alert.	0.803	0.650	0.88	0.69	0.91	− 0.44	− 0.26	− 0.26
4. I use the CPAP treatment because it feels important to me.	0.854	0.731	0.64	0.64	− 0.20	0.13	0.02	− 0.20
5. I use the CPAP treatment because my health is important to me.	0.611	0.385	1.35	1.41	0.36	0.18	− 0.37	− 0.07
6. I use the CPAP treatment because it feels good to use CPAP.	0.682	0.466	1.19	1.13	− 1.73	0.21	0.01	0.37
Factor 2 “Controlled motivation”/items
7. I use the CPAP treatment because other people say I have to.	0.801	0.642	1.06	1.06	− 0.37	0.34	0.09	0.01
8. I use the CPAP treatment because the personnel say I have to.	0.881	0.776	0.76	0.75	− 0.10	0.01	0.27	0.06
9. I use the CPAP treatment because I have to.	0.791	0.625	1.15	1.08	0.47	− 0.36	0.11	0.01

MnSq mean square error, DIF differential item functioning

*Extraction method: Oblimin rotation with Kaiser normalization

**h2 = communalities

***Infit MnSq = information-weighted fit statistic mean square

****Outfit MnSq = outlier-sensitive fit statistic mean square

^aDIF contrast > 0.5 indicates substantial DIF

^bDIF contrast across gender = difficulty for males − difficulty for females

^cDIF contrast across sleepiness = difficulty for patients with scores of the ESS > 10 − difficulty for patients with scores of the ESS ≤ 10

^dDIF contrast across insomnia = difficulty for patients with scores of the MISS > 7 − difficulty for patients with scores of the MISS ≤ 7