Say that there is descriptive data on candidates across election years, districts (or states), and party. The data are currently dis-aggregated at the 'sub-district' level (say, voting precincts).Say that there is descriptive data on candidate
阅读背景：
如何在另一个因子的每一层上聚合一个因子，在分类数据中按另外两个因子分组
来源：互联网
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