Table 1 Differences between participants and non-participants (Leiden University Medical Centre, surgical patient admissions during 2003)

	Participants (n = 1960)	Non-participants (n = 185)	Test of difference
Average age at admission (SD)	52.9 (17.5)	49.5 (21.7)	t = −2.08 p = 0.04
Percentage males	53.5	50.8	χ² = 0.48 p = 0.49
Percentage of patients with procedures performed in operating centre	74.5	73.0	χ² = 0.20 p = 0.65
Percentage with ASA class of 1st operation ⩾3	15.3	20.5	χ² = 6.35 p = 0.10
Percentage with maximum complexity class of 1st operation ⩾5	20.8	14.6	χ² = 4.32 p = 0.23
Percentage with in-hospital adverse outcomes	16.1%	18.9%	χ² = 1.00 p = 0.32
Average duration (days) of admission (SD)	8.8 (14.3)	11.5 (16.8)	t = 2.08 p = 0.04