Classical test theory versus Rasch analysis for quality of life questionnaire reduction

Table 6 Distribution of scores and summary Classical Test Theory (CTT) and Rasch analysis results for the NHP20 and NHP22

	NHP20					NHP22
	Total score	Emotional	Physical	Sleep	Pain	Total score	Physical	Psychological
*Number of items*	20	7	5	4	4	22	11	11
*Principal components results*
Loadings of the first component*	-	0.70	0.68	0.60	0.72	-	0.92	0.92
*Distribution of scores*
Valid observations	7,243	7,382	7,442	7,455	7,452	7,559	7,558	7,557
Mean	35.36	30.45	44.72	40.62	28.24	31.04	29.42	28.21
Standard deviation	29.33	31.46	38.10	39.34	36.50	23.84	28.06	27.40
25^th Percentile	10	0	0	0	0	8.77	0	0
50^th Percentile	30	14.29	40	25	0	28.52	23.56	24.22
75^th Percentile	55	57.14	80	75	50	46.94	48.18	46.46
% 0 score	10.8	30.1	27.0	33.0	49.9	15.7	28.1	26.9
% 100 score	2.3	5.5	17.3	20.3	12.0	1.7	3.0	3.0
*CTT analysis results*
Item-total correlation (range)	0.45–0.65	0.51–0.62	0.57–0.71	0.51–0.75	0.65–0.68	0.46–0.65	0.47–0.68	0.47–0.64
Reliability
Cronbach's α	-	0.82	0.83	0.88	0.87	-	0.88	0.87
Linear combination	0.94	-	-	-	-	0.93	-	-
*Rasch analysis results*
Person separation	2.17	0.74	0.32	0.00	0.00	2.08	1.39	1.24
Person reliability	0.82	0.35	0.09	0.00	0.00	0.81	0.66	0.61

* One component accounts of 67.5% of the total variance for the NHP20, and 85% of the total variance for the NHP22.

ISSN: 1477-7525