1. doBy
官方文档见 CRAN - Package doBy
doBy 主要适用于以下操作：
- 1) Facilities for groupwise computations of summary statistics and other facilities for working with grouped data: ‘do’ something to data stratified ‘by’ some variables.
- 2) LSmeans (least-squares means), general linear contrasts.
- 3) Miscellaneous other utilities.