Table 3: Distribution of groups tested by IRR (Incidence Rate Ratio).

Groups	IRR	Std. Err.	P	95% conf. interval
2	1.08	0.08	0.293	0.94	1.25
3	1.02	0.08	0.812	0.88	1.18
4	1.08	0.08	0.309	0.93	1.24