Table 2.

Extent of between-practice variation explained by each model expressed as percentage reduction in random-effects variance and by median incidence rate ratio

Model	Random-effects variancea	Percentage reduction in variance	MIRRb	Ratio of MIRR to that of null model
Null (random effect only)	0.58	—	2.07	—
Population characteristics only	0.50	16.4	1.95	0.95
Practice characteristics only	0.56	2.8	2.05	0.99
Both population and practice characteristics	0.49	17.9	1.95	0.94

^a This is the variance between practices measured on the log-scale.

^b This is the ratio of the CXR rate for a practice at the 75 th centile of the CXR utilisation against the CXR rate for a practice at the 25 th centile of CXR utilisation, estimated using the random-effects variance. CXR = chest X-ray. MIRR = median incidence rate ratio.