Dear List,
I neeed a grouped list with two sort of categorical data. I have a data
.frame like this.
year cat. b c
1 2006 a1 125 212
2 2006 a2 256 212
3 2005 a1 14 12
4 2004 a3 565 123
5 2004 a2 156 789
6 2005 a1 1 456
7 2003 a2 786 123
8 2003 a1 421 569
9 2002 a2 425 245
I need a list with the sum of b and c for every year and every cat (a1,
a2 or a3) in this year. I had used the tapply function to build the sum
for every year or every cat. How can I combine the two grouping values?
Thanks,
Christian
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