Stratified data with clustering structure, Have some missing values.
A data frame with 956 observations on 4 variables.
[, 1] id cluster id
[, 2] z score
[, 3] group group id
[, 4] stratum stratum id
https://sites.google.com/a/channing.harvard.edu/bernardrosner/channing/incorporating-cluster-effects-for-the-wilcoxon-rank-sum-test-1/stratify–cluswilcox/sample-data