I am trying to: 1. Sample repeated measures data based on subject IDs for cross validation of a GENMOD model, 2. Bootstrap a dataset because of a highly skewed outcome measure recorded in an unbalanced long format (e.g a patient may may have 5 observations for t1, 0 for t2, 3 for t3, and so on, where 't's are time points).   Is there a well explained macro/guide available for resampling or bootstrapping repeated measures data based on subject IDs? I want to resample by ID so that all observations per subject are captured by the procedure in the train/test sets. Please do correct me if I am wrong; I believe simple random sampling by replacement, which may be done using proc surveyselect, would not be an efficient method for repeated measures data. How can I modify proc surveyselect to incorporate subject IDs? ... View more