数据如下：
<br />
df <- data.frame(material=c("w","w","m","m","n","n","w","w","m","m","n","n","w","w","m","m","n","n"),box=c("A","A","A","A","A","A","B","B","B","B","B","B","C","C","C","C","C","C"),value=c("7.2","6.8","9.9","9.6","6.8","6.6","5.5","5.0","6.2","6.1","5.8","5.9","7.8","7.9","9.8","9.9","6.8","6.8"))<br />
df<br />
material box value<br />
1 w A 7.2<br />
2 w A 6.8<br />
3 m A 9.9<br />
4 m A 9.6<br />
5 n A 6.8<br />
6 n A 6.6<br />
7 w B 5.5<br />
8 w B 5.0<br />
9 m B 6.2<br />
10 m B 6.1<br />
11 n B 5.8<br />
12 n B 5.9<br />
13 w C 7.8<br />
14 w C 7.9<br />
15 m C 9.8<br />
16 m C 9.9<br />
17 n C 6.8<br />
18 n C 6.8<br />
如何方便的以box分组，分别计算m和n与w的显著性呢？
就是做一般的t检测~~~[s:18]
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