Table 2.

Incidence of fibromyalgia in age–sex matched group.

	Before PSM (1:4 age–sex matching)		After 1:2 PSM
	Non-AS n = 24,012	ASn = 6,003	Non-AS n= 11,392	ASn = 5,696
Follow up person months	1,913,078	471,482	905,332	447,988
New fibromyalgia case	607	239	356	232
Incidence rate*(95% CI)	0.32 (0.29–0.34)	0.51 (0.45–0.58)	0.39 (0.35–0.44)	0.52 (0.46–0.59)
Crude Relative risk (95% CI)	Reference	1.60 (1.37–1.85)	Reference	1.32 (1.12–1.55)

^*Incidence rate, per 1,000 person-months.