Table 3.

Item parameters for the 7-item and reduced 4-item 2-PL IRT models

Item		7-item					4-item
		Ba	SEb	Zc	#P	95% CI	Ba	SEb	Zc	#P	95% CI
1	Daytime freq
	Discrimination	2.52	0.47	5.37	0.000	1.60–3.44	2.78	0.47	5.89	0.00	1.85–3.70
	Difficulty	0.65	0.08	8.24	0.000	0.49–0.80	0.64	0.08	8.43	0.00	0.49–0.79
2	Nocturnal freq
	Discrimination	2.07	0.35	5.96	0.00	1.39–2.75	2.16	0.37	5.83	0.00	1.43–2.88
	Difficulty	1.17	0.12	10.05	0.00	0.94–1.40	1.17	0.10	11.38	0.00	0.97–1.38
3	Urgency
	Discrimination	1.78	0.31	5.69	0.00	1.17–2.39	1.52	0.29	5.32	0.00	0.96–2.08
	Difficulty	1.55	0.12	9.33	0.00	1.23–1.88	1.70	0.21	8.15	0.00	1.29–2.10
4	Bleeding
	Discrimination	1.55	0.23	6.69	0.00	1.09–2.00	1.50	0.23	6.51	0.00	1.05–1.95
	Difficulty	0.63	0.10	6.52	0.00	0.44–0.82	0.65	0.10	6.57	0.00	0.46–0.85
5	Well-being
	Discrimination	1.73	0.31	5.64	0.00	1.13–2.33
	Difficulty	1.57	0.17	9.11	0.00	1.23–1.91
6	Arthritis
	Discrimination	0.41	0.16	2.53	0.01	0.09–0.73
	Difficulty	4.21	1.58	2.65	0.01	1.10–7.32
7	Uveitis
	Discrimination	0.61	0.25	2.45	0.01	0.12–1.09
	Difficulty	4.85	1.80	2.69	0.01	1.32–8.39

^aB = standard beta coefficient

^bSE = standard error of estimation

^cZ = z-score

^#P = p > x (exact p-value)