Development and Validation of the “Keratoconus End-Points Assessment Questionnaire” (KEPAQ), a Disease-Specific Instrument for Evaluating Subjective Emotional Distress and Visual Function Through Rasch Analysis

Figures & data

Table 1 Table for Transforming KEPAQ-E Raw Score to Person Measure, Which Is the Value That Should Be Used for Epidemiological and Clinical Applications, According to Rasch Analysis Theory

Raw Score	Person Measure Equivalent	Standard Error
0	–5.47	1.89
1	–4.12	1.10
2	–3.22	0.83
3	–2.64	0.71
4	–2.20	0.63
5	–1.83	0.58
6	–1.51	0.55
7	–1.21	0.54
8	–0.93	0.53
9	–0.65	0.53
10	–0.37	0.53
11	–0.09	0.54
12	0.21	0.56
13	0.53	0.58
14	0.88	0.60
15	1.26	0.64
16	1.70	0.69
17	2.22	0.76
18	2.87	0.86
19	3.72	0.99
20	4.89	1.21
21	6.40	1.93

Table 2 Table for Transforming KEPAQ-F Raw Score to Person Measure, Which Is the Value That Should Be Used for Epidemiological and Clinical Applications, According to Rasch Analysis Theory

Raw Score	Person Measure Equivalent	Standard Error
0	–5.43	1.84
1	–4.19	1.03
2	–3.45	0.75
3	–2.98	0.63
4	–2.63	0.56
5	–2.33	0.52
6	–2.07	0.50
7	–1.84	0.48
8	–1.61	0.47
9	–1.40	0.46
10	–1.18	0.46
11	–0.97	0.47
12	–0.75	0.47
13	–0.52	0.48
14	–0.28	0.50
15	–0.02	0.51
16	0.24	0.53
17	0.54	0.56
18	0.86	0.58
19	1.21	0.60
20	1.59	0.63
21	2.00	0.66
22	2.47	0.70
23	3.00	0.76
24	3.63	0.83
25	4.41	0.94
26	5.49	1.18
27	6.97	1.93

Table 3 Psychometric Properties of the Final Versions of Both Sub-Scales for the KEPAQ

	Rasch Model Expectation	KEPAQ-E	KEPAQ-F
Number of items		7	9
Response categories	Ordered	Ordered	Ordered
PSI	>2.00	2.43	3.19
Number of misfitting items	0	0	0
Variance explained by measure	>50%	62.48%	61.30%
Eigenvalue First PCA contrast	<3.0	1.43	1.98
DIF	Non-significant	Non-significant	Non-significant

Abbreviations: PSI, Person Separation Index; PCA, principal component analysis of residuals; DIF, differential item functioning.

Figure 1 Emotional Compromise sub-scale of the Keratoconus End-Points Assessment Questionnaire (KEPAQ-E).

Figure 2 Functional Compromise sub-scale of the Keratoconus End-Points Assessment Questionnaire (KEPAQ-F).