Table 2.

Results of Polyp Prevention Trial

stratum s			adenoma			difference in observed	weight	bias factor ε(max)s
stratum s			recurrence			rates of recurrence ds	ws
sex	age	group	no	yes	missing
		control	573	374	94 (9%)
		study	578	380	76 (7%)
men	30–49	control	33	22	5 (8%)	-.23	.07	.09
		study	58	12	3 (4%)
	40–59	control	99	76	7 (4%)	.01	.17	.05
		study	94	76	9 (5%)
	60–69	control	122	105	25 (10%)	-.04	.23	.11
		study	144	105	18 (7%)
	70–79	control	65	76	26 (16%)	-.04	.13	.20
		study	70	71	29 (17%)
women	30–49	control	54	11	3 (4%)	.03	.10	.07
		study	47	12	4 (6%)
	40–59	control	69	24	4 (4%)	.02	.11	.04
		study	69	27	4 (4%)
	60–69	control	77	31	13(11%)	.08	.12	.11
		study	68	40	5 (4%)
	70–79	control	54	29	11(12%)	.22	.07	.12
		study	28	37	4 (6%)

The overall estimate of the difference in probabilities of recurrence between study and control groups is = Σ_sd_sw_s = -.003 with a standard error .022. We define ε_(max)s = max((1 - π_0s)/π_1s, (1 - π_1s)/π_0s), where π_zs equals one minus the fraction missing in group z and stratum s. The anticipated maximum bias is ψ_max Σ_s ε_(max)s w_s = ± .10 ψ_max, where ψ_max is the anticipated bias if there were complete confounding of the unobserved covariate and treatment.