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library(MASS)
help(birthwt)

## starting httpd help server ... done

table(birthwt$race)

## 
##  1  2  3 
## 96 26 67

c("White", "Black", "Other")[birthwt$race]

##   [1] "Black" "Other" "White" "White" "White" "Other" "White" "Other" "White"
##  [10] "White" "Other" "Other" "Other" "Other" "White" "White" "Black" "White"
##  [19] "Other" "White" "Other" "White" "White" "Other" "Other" "White" "White"
##  [28] "White" "Black" "Black" "Black" "White" "Black" "White" "Black" "White"
##  [37] "White" "White" "White" "White" "Black" "White" "Black" "White" "White"
##  [46] "White" "White" "Other" "White" "Other" "White" "Other" "White" "White"
##  [55] "Other" "Other" "Other" "Other" "Other" "Other" "Other" "Other" "Other"
##  [64] "White" "Other" "Other" "Other" "Other" "White" "Black" "White" "Other"
##  [73] "Other" "Black" "White" "Black" "White" "White" "Black" "White" "White"
##  [82] "White" "Other" "Other" "Other" "Other" "Other" "White" "White" "White"
##  [91] "White" "Other" "White" "White" "White" "White" "White" "White" "White"
## [100] "White" "White" "White" "Other" "White" "Other" "Black" "White" "White"
## [109] "White" "Black" "White" "Other" "White" "White" "White" "Other" "White"
## [118] "Other" "White" "Other" "White" "Other" "White" "White" "White" "White"
## [127] "White" "White" "White" "White" "Other" "White" "Black" "Other" "Other"
## [136] "Other" "Other" "Black" "Other" "White" "White" "White" "Other" "Other"
## [145] "White" "White" "Black" "White" "Other" "Other" "Other" "White" "White"
## [154] "White" "White" "Other" "Black" "White" "Black" "Other" "White" "Other"
## [163] "Other" "Other" "Black" "White" "Other" "Other" "White" "White" "Black"
## [172] "Black" "Black" "Other" "Other" "White" "White" "White" "White" "Black"
## [181] "Other" "Other" "White" "Other" "White" "Other" "Other" "Black" "White"

There are 26 black mothers

The code c(“White”, “Black”, “Other”)[birthwt$race] show us the corresponding labels