Table 4.

Summary of the results from the invariance testing of the questionnaires used to measure the symptoms of CFS/ME and the FM groups

Questionnaire	χ2	df	∆χ2	∆df	P value	CFI	RMSEA	Invariance confirmed
American CDC Symptom Inventory	63.20	30	18.62	6	0.01	0.85	0.07	No
FIQ	1.74	71	29.49	9	0.01	0.92	0.06	No
MPQ	40.83	30	2.59	6	0.86	0.95	0.04	Yes
MFI	24.57	14	1.87	4	0.76	0.98	0.06	Yes
PSQI	40.83	30	2.59	6	0.86	0.95	0.04	Yes
SF-36 V2	116.26	43	13.14	9	0.09	0.89	0.09	Yes
HADS	223.81	160	21.39	12	0.04	0.94	0.04	No
MHLOC form C	420.89	366	14.01	14	0.45	0.84	0.05	Yes
RSES	122.12	79	4.88	9	0.85	0.95	0.05	Yes
Syndrome Model	27.69	12	10.49	4	0.07	0.91	0.08	Yes

CDC, Centers for Disease Control and Prevention; CFI, Comparative Fit Index; CFS, chronic fatigue syndrome; FIQ, Fibromyalgia Impact Questionnaire; FM, fibromyalgia; HADS, Hospital Anxiety and Depression Scale; ME, myalgic encephalomyelitis; MFI, Multidimensional Fatigue Inventory; MHLOC, Multidimensional Health Locus of Control Scale; MPQ, McGill Pain Questionnaire; PSQI, Pittsburgh Sleep Quality Index; RMSEA, root mean square error of approximation; RSES, Rosenberg Self-Esteem Scale.