Creating new variables

Jeffrey Leek
Johns Hopkins Bloomberg School of Public Health

Why create new variables?

  • Often the raw data won't have a value you are looking for
  • You will need to transform the data to get the values you would like
  • Usually you will add those values to the data frames you are working with
  • Common variables to create
    • Missingness indicators
    • "Cutting up" quantitative variables
    • Applying transforms

Example data set

Getting the data from the web

if(!file.exists("./data")){dir.create("./data")}
fileUrl <- "https://data.baltimorecity.gov/api/views/k5ry-ef3g/rows.csv?accessType=DOWNLOAD"
download.file(fileUrl,destfile="./data/restaurants.csv",method="curl")
restData <- read.csv("./data/restaurants.csv")

Creating sequences

Sometimes you need an index for your data set

s1 <- seq(1,10,by=2) ; s1

[1] 1 3 5 7 9

s2 <- seq(1,10,length=3); s2

[1]  1.0  5.5 10.0

x <- c(1,3,8,25,100); seq(along = x)

[1] 1 2 3 4 5

Subsetting variables

restData$nearMe = restData$neighborhood %in% c("Roland Park", "Homeland")
table(restData$nearMe)


FALSE  TRUE 
 1314    13

Creating binary variables

restData$zipWrong = ifelse(restData$zipCode < 0, TRUE, FALSE)
table(restData$zipWrong,restData$zipCode < 0)


        FALSE TRUE
  FALSE  1326    0
  TRUE      0    1

Creating categorical variables

restData$zipGroups = cut(restData$zipCode,breaks=quantile(restData$zipCode))
table(restData$zipGroups)


(-2.123e+04,2.12e+04]  (2.12e+04,2.122e+04] (2.122e+04,2.123e+04] (2.123e+04,2.129e+04] 
                  337                   375                   282                   332 

table(restData$zipGroups,restData$zipCode)


                        -21226 21201 21202 21205 21206 21207 21208 21209 21210 21211 21212 21213
  (-2.123e+04,2.12e+04]      0   136   201     0     0     0     0     0     0     0     0     0
  (2.12e+04,2.122e+04]       0     0     0    27    30     4     1     8    23    41    28    31
  (2.122e+04,2.123e+04]      0     0     0     0     0     0     0     0     0     0     0     0
  (2.123e+04,2.129e+04]      0     0     0     0     0     0     0     0     0     0     0     0

                        21214 21215 21216 21217 21218 21220 21222 21223 21224 21225 21226 21227
  (-2.123e+04,2.12e+04]     0     0     0     0     0     0     0     0     0     0     0     0
  (2.12e+04,2.122e+04]     17    54    10    32    69     0     0     0     0     0     0     0
  (2.122e+04,2.123e+04]     0     0     0     0     0     1     7    56   199    19     0     0
  (2.123e+04,2.129e+04]     0     0     0     0     0     0     0     0     0     0    18     4

                        21229 21230 21231 21234 21237 21239 21251 21287
  (-2.123e+04,2.12e+04]     0     0     0     0     0     0     0     0
  (2.12e+04,2.122e+04]      0     0     0     0     0     0     0     0
  (2.122e+04,2.123e+04]     0     0     0     0     0     0     0     0
  (2.123e+04,2.129e+04]    13   156   127     7     1     3     2     1

Easier cutting

library(Hmisc)
restData$zipGroups = cut2(restData$zipCode,g=4)
table(restData$zipGroups)


[-21226,21205) [ 21205,21220) [ 21220,21227) [ 21227,21287] 
           338            375            300            314

Creating factor variables

restData$zcf <- factor(restData$zipCode)
restData$zcf[1:10]

 [1] 21206 21231 21224 21211 21223 21218 21205 21211 21205 21231
32 Levels: -21226 21201 21202 21205 21206 21207 21208 21209 21210 21211 21212 21213 21214 ... 21287

class(restData$zcf)

[1] "factor"

Levels of factor variables

yesno <- sample(c("yes","no"),size=10,replace=TRUE)
yesnofac = factor(yesno,levels=c("yes","no"))
relevel(yesnofac,ref="yes")

 [1] yes yes yes yes no  yes yes yes no  no 
Levels: yes no

as.numeric(yesnofac)

 [1] 1 1 1 1 2 1 1 1 2 2

Cutting produces factor variables

library(Hmisc)
restData$zipGroups = cut2(restData$zipCode,g=4)
table(restData$zipGroups)


[-21226,21205) [ 21205,21220) [ 21220,21227) [ 21227,21287] 
           338            375            300            314

Using the mutate function

library(Hmisc); library(plyr)
restData2 = mutate(restData,zipGroups=cut2(zipCode,g=4))
table(restData2$zipGroups)


[-21226,21205) [ 21205,21220) [ 21220,21227) [ 21227,21287] 
           338            375            300            314

Common transforms

Notes and further reading