h the levels "male" and "female", this method would calculate four Kaplan-Meier curves (Treatment + male, Treatment + female, Control + male, Control + female). Next a simple weighted average of these survival curves is taken per level in <code>variable</code>, where the weights are the number of occurences of each confounder level in the reference data. The reference data is the pooled sample by default, but external reference data can be used. A more detailed description can be found in the original artic