Use help to examine the coding scheme for the mother’s race variable in the birthwt{MASS} dataset. The MASS comes with the base R installation but is not automatically loaded when R is invoked. How many black mothers are there in this data frame? What does the following R command do? c(“White”, “Black”, “Other”)[birthwt$race] ## birthwt{MASS}

library(MASS)
data(birthwt, package = "MASS")
dta<-birthwt
head(dta)
##    low age lwt race smoke ptl ht ui ftv  bwt
## 85   0  19 182    2     0   0  0  1   0 2523
## 86   0  33 155    3     0   0  0  0   3 2551
## 87   0  20 105    1     1   0  0  0   1 2557
## 88   0  21 108    1     1   0  0  1   2 2594
## 89   0  18 107    1     1   0  0  1   0 2600
## 91   0  21 124    3     0   0  0  0   0 2622
str(dta) 
## 'data.frame':    189 obs. of  10 variables:
##  $ low  : int  0 0 0 0 0 0 0 0 0 0 ...
##  $ age  : int  19 33 20 21 18 21 22 17 29 26 ...
##  $ lwt  : int  182 155 105 108 107 124 118 103 123 113 ...
##  $ race : int  2 3 1 1 1 3 1 3 1 1 ...
##  $ smoke: int  0 0 1 1 1 0 0 0 1 1 ...
##  $ ptl  : int  0 0 0 0 0 0 0 0 0 0 ...
##  $ ht   : int  0 0 0 0 0 0 0 0 0 0 ...
##  $ ui   : int  1 0 0 1 1 0 0 0 0 0 ...
##  $ ftv  : int  0 3 1 2 0 0 1 1 1 0 ...
##  $ bwt  : int  2523 2551 2557 2594 2600 2622 2637 2637 2663 2665 ...

結論： 1.birthw屬於data.frame 2.有189觀察值，共10個變項

class(dta$race)
## [1] "integer"
typeof(dta$race)
## [1] "integer"
factor(dta$race)
##   [1] 2 3 1 1 1 3 1 3 1 1 3 3 3 3 1 1 2 1 3 1 3 1 1 3 3 1 1 1 2 2 2 1 2 1 2 1 1
##  [38] 1 1 1 2 1 2 1 1 1 1 3 1 3 1 3 1 1 3 3 3 3 3 3 3 3 3 1 3 3 3 3 1 2 1 3 3 2
##  [75] 1 2 1 1 2 1 1 1 3 3 3 3 3 1 1 1 1 3 1 1 1 1 1 1 1 1 1 1 3 1 3 2 1 1 1 2 1
## [112] 3 1 1 1 3 1 3 1 3 1 3 1 1 1 1 1 1 1 1 3 1 2 3 3 3 3 2 3 1 1 1 3 3 1 1 2 1
## [149] 3 3 3 1 1 1 1 3 2 1 2 3 1 3 3 3 2 1 3 3 1 1 2 2 2 3 3 1 1 1 1 2 3 3 1 3 1
## [186] 3 3 2 1
## Levels: 1 2 3

結論： 1.race屬於integer(整數) 2.以factor「類別變數」表示資料型態，共分為1,2,3三種(category variable)

dta$n_race<-c("White", "Black", "Other")[dta$race]
class(dta$n_race)
## [1] "character"
typeof(dta$n_race)
## [1] "character"
factor(dta$n_race)
##   [1] Black Other White White White Other White Other White White Other Other
##  [13] Other Other White White Black White Other White Other White White Other
##  [25] Other White White White Black Black Black White Black White Black White
##  [37] White White White White Black White Black White White White White Other
##  [49] White Other White Other White White Other Other Other Other Other Other
##  [61] Other Other Other White Other Other Other Other White Black White Other
##  [73] Other Black White Black White White Black White White White Other Other
##  [85] Other Other Other White White White White Other White White White White
##  [97] White White White White White White Other White Other Black White White
## [109] White Black White Other White White White Other White Other White Other
## [121] White Other White White White White White White White White Other White
## [133] Black Other Other Other Other Black Other White White White Other Other
## [145] White White Black White Other Other Other White White White White Other
## [157] Black White Black Other White Other Other Other Black White Other Other
## [169] White White Black Black Black Other Other White White White White Black
## [181] Other Other White Other White Other Other Black White
## Levels: Black Other White

結論： 1.創造dta$n_race新的變數，以race 1=“White”, 2=“Black”, 3=“Other”帶入 2.race屬於character 3.以factor「類別變數」表示資料型態，共分為Black, Other, White三種(會以英文字母順序排列)

table(dta$n_race)
## 
## Black Other White 
##    26    67    96

結論： 1.黑人有26個。