Table 3.

Results from analyses of plasmode simulation using various methods to account for immortal time.

Model	HRa	log SE	95% CICb	Bias (log scale)c
Dichotomous exposure with misclassified immortal time	0.84	0.029	0.000	−0.177
Dichotomous exposure with exclusion of immortal person-time	0.93	0.030	0.348	−0.074
Time-varying exposure
Ever/never treated	1.02	0.030	0.919	0.016
8-month treatment window	1.01	0.032	0.946	0.012
12-month treatment window	1.01	0.031	0.938	0.014
Landmark model
8-month landmark	1.00	0.038	0.946	−0.001
12-month landmark	1.00	0.038	0.955	−0.001
Clone-censor-weight method
8-month treatment window	1.00	0.029	0.949	0.001
12-month treatment window	1.00	0.028	0.948	−0.003

Abbreviation: CIC, confidence interval coverage.

^aMean HR across all 1,000 plasmode simulation datasets.

^bProportion of 95% CIs that include the true HR of 1.00.

^cDifference between the log average HR across the 1,000 plasmode datasets and the true log HR (0); larger absolute numbers indicate more bias. Negative bias makes surgical resection appear more protective, positive bias makes surgical resection appear more harmful. In our example, since the true log HR is 0, bias equals the log of the average HR. In simulations where the true HR is not null (1.00), this will not be the case.