Below you see the full data set bcmort (from the ISwR package). The four-level factor cohort can be considered the product of two two-level factors:
period with levels 1991-2001 and 1981-1991area with levels Copenhagen and National
See the details in help(bcmort). Create a data frame bcmort2 with those two factors added to the existing variables.
age cohort bc.deaths p.yr
1 50-54 Study gr. 9 64144
2 55-59 Study gr. 34 92734
3 60-64 Study gr. 43 83510
4 65-69 Study gr. 53 87408
5 70-74 Study gr. 56 77427
6 75-79 Study gr. 28 25600
7 50-54 Nat.ctr. 89 767111
8 55-59 Nat.ctr. 434 1067778
9 60-64 Nat.ctr. 516 906943
10 65-69 Nat.ctr. 535 826254
11 70-74 Nat.ctr. 545 635385
12 75-79 Nat.ctr. 214 192946
13 50-54 Hist.ctr. 22 57669
14 55-59 Hist.ctr. 45 113143
15 60-64 Hist.ctr. 82 139065
16 65-69 Hist.ctr. 104 155697
17 70-74 Hist.ctr. 128 128454
18 75-79 Hist.ctr. 57 40196
19 50-54 Hist.nat.ctr. 104 577528
20 55-59 Hist.nat.ctr. 443 931245
21 60-64 Hist.nat.ctr. 485 916923
22 65-69 Hist.nat.ctr. 491 838476
23 70-74 Hist.nat.ctr. 418 608008
24 75-79 Hist.nat.ctr. 182 182824
(from: Dalgaard, 2008, Introductory Statistics with R, Springer)