. Author manuscript; available in PMC: 2015 Aug 7.

Published in final edited form as: Accid Anal Prev. 2012 Aug 11;50:778–786. doi: 10.1016/j.aap.2012.07.002

Table 4.

Incidence-rate ratios (IRR) and 95% confidence intervals (CI) for random effects negative binomial regression: most serious injuries reported in the 5669 mine-year records used.

	IRR	Std. err.	p > z	[95% CI]
Ln real revenue per hour worked	0.841	0.045	0.001	0.757	0.934
Mine age	0.947	0.007	0.000	0.933	0.962
Union status (union = 1)	0.960	0.077	0.608	0.820	1.123
Mining method (longwall = 1)	0.509	0.067	0.000	0.393	0.659
Region
Central (reference)
North	1.366	0.174	0.014	1.064	1.753
Midwest	0.682	0.098	0.008	0.515	0.904
West	1.500	0.297	0.041	1.017	2.212
South	0.655	0.145	0.055	0.425	1.009
Total hours worked			(Exposure)
/ln_r	0.518	0.076		0.370	0.666
/ln_s	−0.004	0.109		−0.218	0.211
R	1.679	0.127		1.448	1.947
S	0.996	0.109		0.804	1.235
Number of observations			5669
Number of groups			1407
Obs. per group
Min			1
Max			17
Average			4
Wald χ² (8)			191.64
Log likelihood (prob > χ²)			−4405 (0.000)
Pseudo R²			0.21