Table 4

Numbers of new lung cancer diagnoses and x-rays (and practice populations) and incidence rates in the 12 months before (March 2007–February 2008) and 12 months after (March 2008–February 2009) the intervention

	Outcome	Area		IRR Int versus Cont	95% CI for IRR		p Value
	Outcome	Control (n=9 practices)	Intervention (n=11 practices)	IRR Int versus Cont	95% CI for IRR		p Value
Time
Before	Number of lung cancer diagnoses	47	59
	Population at risk	43 444	67 119
	Rate/100 000 population/year	108	88	0.81	0.55	1.19	0.288
After	Number of lung cancer diagnoses	42	75
	Population at risk	43 444	67 119
	Rate/100 000 population/year	97	112	1.16	0.79	1.69	0.452
Interaction^*				1.42	0.83	2.44	0.199

Time
Before	Number of x-rays	1605	2875
	Population at risk	43 444	67 119
	Rate/100 000 population/year	3690	4280	1.16	1.09	1.23	0.001
After	Number of x-rays	1573	3442
	Population at risk	43 444	67 119
	Rate/100 000 population/year	3620	5130	1.42	1.33	1.50	0.001
Interaction^*				1.22	1.12	1.33	0.001

↵* The interaction term represents the change over time in the outcome (ie, after minus before) for the intervention area minus the same effect for the control area.
Cont, control; Int, intervention; IRR, incidence rate ratio.