Table 3.

Item characteristics of the SMQ; number of valid responses (n), item correlations with sum score (r) and Cronbach’s alpha when the item is dropped (α)

Item	n	r	α
Item 1	283	0.84	0.96
Item 2	285	0.27	0.97
Item 3	285	0.83	0.96
Item 4	284	0.88	0.96
Item 5	283	0.92	0.96
Item 6	279	0.88	0.96
Item 7	284	0.72	0.96
Item 8	285	0.72	0.96
Item 9	276	0.78	0.96
Item 10	281	0.56	0.96
Item 11	284	0.81	0.96
Item 12	152	0.78	0.96
Item 13	284	0.84	0.96
Item 14	283	0.87	0.96
Item 15	283	0.89	0.96
Item 16	282	0.89	0.96
Item 17	278	0.92	0.96

Item characteristics of the SMQ symptom scale in the total sample (clinical group + control group)

n = number of valid responses

r = item correlation with sum score on the SMQ symptom scale

α = Cronbach’s alpha when the item is dropped