ما معنى Fama–MacBeth regression, clustered standard errors؟

A Fama–MacBeth regression (Fama and MacBeth, 1973) runs one cross-sectional regression per period and reports the time-series mean of the slopes, with standard error their standard deviation over $\sqrt T$. Clustered standard errors compute the sandwich variance with the scores summed within clusters (firms, or periods), allowing any correlation inside a cluster and none across clusters.