Confounding and Simpson's paradox

Details ofreconstructive surgery and amputations carried out in districts with below (low volume) and above (high volume) the average rate of arterial reconstructions in the Oxford region.Values are numbers or proportions (percentages) Statistics High volume Low volume x2 df P value Referrals from outside the district 10/67 (15) 2/46 (4 3) 2-197 1 0-138 Balloon angioplasty before reconstructive surgery 23/67 (34) 3/41 (7-3) 8-729 1 0-0031 Types of infrainguinal graft: Femorotibial graft 17 (25) 4 (11) Femoropopliteal: 1 23-943 2 00001 Vein 31 (46) 4 (11) 293 2 000 Prosthesis 19 (28) 27 ( 77) Amputation rate per 100 000 population peryear (95% confidence interval) 6-1 (40 to 8 2) 10-3 (7-4to 13-2) 5-365 1 0-021 Ratio ofbelow knee to above knee amputations