对于MASS包内建数据painters, 如下所示



求学校C作品Composition得分的平均数。

方法一
1.step 找出logical index vector

> library(MASS)                 # load the MASS package
> school = painters$School      # the painter schools
> c_school = school == "C"      # the logical index vector




2.step 找出school为C的行

> c_painters = painters[c_school, ]


3.step 计算平均数


> mean(c_painters$Composition)
[1] 13.167


方法二
直接用tapply，算出所有学校的Composition的分数平均数。


> tapply(painters$Composition, painters$School, mean)
     A      B      C      D      E      F      G      H
10.400 12.167 13.167  9.100 13.571  7.250 13.857 14.000



http://www.r-tutor.com/elementary-statistics/qualitative-data/category-statistics