Dear all, I am using the "svykm" function (in the survey package) to estimate survival in a two-phase study. I would like to know if there is any published reference for the estimator of the variance of survival curve under general sampling in order to cite it? The code uses the linearization strategy by Williams (Product-Limit Survival Functions with Correlated Survival Times,Lifetime Data Analysis 1995), but I found the formulas for general complex sampling in a technical note written by Thomas Lumley (http://staff.washington.edu/tlumley/survey/survcurve.pdf) and I was wondering if it was ever fomally validated and published. Thanks, Paola [[alternative HTML version deleted]]