Clustered or grouped errors are errors that are correlated within a cluster or group and are uncorrelated across clusters.
A simple example of clustering arises when sampling is of independent units but errors for individuals within the unit are correlated.
这个能不能这么说。在两个省之间抽样,省之间是独立的,但是两省中的个体说不定会有联系?怎么感觉cluster的种类很多呢?