We used multiple imputation for the main analysis to take account of this.
The conditional logistic regression model was used for the main analysis to take into account the matched design.
Therefore, our main analysis compared CD133- to CD133+ fractions accounting for cell of origin (Methods).
In Figure 2, the 38 populations (PRW being excluded) were placed according to the two main axes of the correspondence analysis (accounting for 27.4% and 11.5% of the inertia, respectively).
Thirdly, we repeated our main analysis using robust standard errors to account for any possible clustering within conurbations.
Estimates from multilevel models that account for clustering at the TPU and district levels were similar to the main analysis (see Supplemental Material, Table S3), as were estimates from models that account for spatial autocorrelation (see Supplemental Material, Table S4).
SCC, the most common histologic type of oropharyngeal cancer, accounted for 557 (94.7%) of all cases, and the main analysis was restricted to these cases (Table 2).
The mixed effects analysis, which accounted for differences between hospitals, produced results consistent with those of the main analysis (regional v general anesthesia 0.91, 0.75 to 1.10, and combined v general anesthesia 0.98, 0.79 to 1.21).
These descriptive differences were taken into account for the determination of the adjustment factors for the main analysis.
To explore whether the results from the main analysis remain when taking the different lengths of time between the visits into account, the number of days and its square are added as control variables in all regressions.
