Table 7.

The volume–mortality relationship

	OR	95 % CI	p value
Overall: 100 case/year increase*	0.84	(0.78–0.9)	< 0.0001
Risk category by volume (p = 0.0049 for interaction)
100 C/Y increase: risk category 1	0.98	(0.78–1.23)	0.8644
100 C/Y increase: risk category 2	0.74	(0.66–0.83)	< 0.0001
100 C/Y increase: risk category 3	0.89	(0.82–0.97)	0.0059
100 C/Y increase: risk category 4	0.85	(0.77–0.93)	0.0012
100 C/Y increase: risk category 5&6	0.77	(0.69–0.86)	< 0.0001
Time period by volume (p = 0.0023 for interaction)
100 C/Y increase: 1982–1987	0.83	(0.73–0.95)	0.0063
100 C/Y increase: 1988–1992	0.77	(0.69–0.87)	< 0.0001
100 C/Y increase: 1993–1997	0.77	(0.70–0.85)	< 0.0001
100 C/Y increase: 1998–2002	0.93	(0.85–1.01)	0.0994
100 C/Y increase: 2003–2007	0.78	(0.69–0.88)	0.0002
Age group by volume (p = 0.3494 for interaction)
100 C/Y increase: neonates	0.85	(0.79–0.93)	0.0003
100 C/Y increase: infants	0.84	(0.77–0.92)	0.0002
100 C/Y increase: children	0.80	(0.72–0.88)	< 0.0001

^*Values are derived from four separate models (1 main effect model and 3 interaction models), each controlling for correlation within the center but not within the patients

OR odds ratio, CI confidence interval, C/Y cases per year