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I was looking at the help page for the aggregate function in R. I had never used this convenience function but I have a process it should help me speed up. However, I've been totally unable to walk through the example and understand what is going on. One example is the following: 1> aggregate(state.x77, list(Region = state.region), mean) Region Population Income Illiteracy Life Exp Murder HS Grad Frost Area 1 Northeast 5495 4570 1.000 71.26 4.722 53.97 132.78 18141 2 South 4208 4012 1.738 69.71 10.581 44.34 64.62 54605 3 North Central 4803 4611 0.700 71.77 5.275 54.52 138.83 62652 4 West 2915 4703 1.023 71.23 7.215 62.00 102.15 134463 The output here is exactly what I would expect. So I try to understand what is going on. So I look at state.x77 1> head(state.x77) Population Income Illiteracy Life Exp Murder HS Grad Frost Area Alabama 3615 3624 2.1 69.05 15.1 41.3 20 50708 Alaska 365 6315 1.5 69.31 11.3 66.7 152 566432 Arizona 2212 4530 1.8 70.55 7.8 58.1 15 113417 Arkansas 2110 3378 1.9 70.66 10.1 39.9 65 51945 California 21198 5114 1.1 71.71 10.3 62.6 20 156361 Colorado 2541 4884 0.7 72.06 6.8 63.9 166 103766 OK, that's odd to me. I would expect to see a column in state.x77 named state.region or something. So state.region must be its own object. So I do a str() on it: 1> str(state.region) Factor w/ 4 levels "Northeast","South",..: 2 4 4 2 4 4 1 2 2 2 ... It looks like state.region is just a factor. Somehow there HAS to be a connection between state.region and state.x77 in order for aggregate() to group state.x77 b

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