Table 2.

Model associations with incidence of initial respiratory morbidiy counts, in 122 pens of cattle on 102 feedlots

Variable	IRR	SEa	Pb	95% CI
Gender category
Steers	Referent
Heifers	1.3	0.3	0.28	0.8, 2.0
Mixed heifers and steers	3.7	1.25	0.00	1.9, 7.2
Multiple sources (Y/N)	2.0	0.51	.008	1.2, 3.3
Pen average miles transported	1.001	0.0003	0.001	1.0004, 1.002
Entry weight
< 250 kg (550 lb)	Referent
250–318 kg (550–700 lb)	0.52	0.2	0.09	0.25, 1.1
> 318 kg (700 lb)	0.18	0.07	0.00	0.08, 0.38
Weeks on feed
Weeks 1–3	Referent
Weeks 4–7	0.44	0.08	0.00	0.31, 0.61
Weeks 8–12	0.13	0.03	0.00	0.09, 0.19

^aSemi-Robust Huber/White sandwich estimated standard errors

^bWald Chi-square statistics

Dispersion 1.05