Dear experts, I am asking for help with a question regarding to stratified bootstrap. How shall I implement my stratified bootstrap in boot() using boot package in R? My dataset is a longitudinal dataset (3 measurements per person at year 1, 4 and 7) composed of multiple clinic centers and multiple participants within each clinic. It has missing values. I want to do a bootstrap to find the standard errors and confidence intervals for my variance components. My model is a mixed model with random clinic and random participant within clinic. I thought two methods to do bootstrap: (1) bootstrap data; however, I have problem specifying the second parameter for my statistic function, shall I use indices, weight or frequency and how shall I relate to my dataset. (2) bootstrap residuals; however, the dataset has multiple measurements and missing values. I am wondering how to construct a new data frame containing the residuals and fitted values. Your help will be highly appreciated! Sincerely yours, Qian