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This is a nice overview of how to use R to adjust survival curves in an observational study. It covers weighting and modeling with covariates and criticizes several approaches.

Terry M. Thernau, Cynthia S. Crawson, Elizabeth J. Atkinson. Adjusted survival curves. 2015 (January). Available at https://cran.r-project.org/web/packages/survival/vignettes/adjcurve.pdf.