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Stata Class Notes 3.0
Modifying Data

1.0 Stata commands in this unit

2.0 Demonstration and explanation

use http://www.ats.ucla.edu/stat/stata/notes3/hs0

Let's use the codebook command to see what our variables look like.  Because we have not listed any variables after the command, Stata will show us the codebook for all of the variables.

codebook

First, let's order the variables in a way that makes sense.  While there are several possible orderings that are logical, we will put the id variable first, followed by the demographic variables, such as gender, ses and program type.  We will put the variables regarding the test scores at the end.

order id gender 

Now let's include some variable labels so that we know a little more about the variables.

label variable schtyp "The type of school the student attended."
label define scl 1 public 2 private
label values schtyp scl
codebook schtyp
list schtyp in 1/10
list schtyp in 1/10, nolabel

Now let's create a new numeric version of the string variable prgtype.  We will call our new variable prog.

encode prgtype, gen(prog)
label variable prog "The type of program in which the student was enrolled."
codebook prog
list prog in 1/10
list prog in 1/10, nolabel

The variable gender may give us trouble in the future because it is difficult to know what the 0s and 1s mean.

rename gender female
label variable female "The gender of the student."
label define fm 1 female 0 male
label values female fm
codebook female
list female in 1/10
list female in 1/10, nolabel

Let's recode the value 5 in the variable race to be missing.

list race if race == 5
recode race 5 = .
list race if race == .

Now let's create a variable that is a total of some of the test scores.

generate total = read + write + math
summarize total

It might make more sense to add the social studies score to the total rather than the math score, so let's change that.

replace total = read + write + socst
summarize total
label variable total "The total of the read, write and socst."
codebook total

Now let's see if we can assign some letter grades to these test scores.

recode total (0/80=0 F) (80/110=1 D) (110/140=2 C) (140/170=3 B) (170/300=4 A), gen(grade)
codebook grade
label variable grade "These are the combined grades of read, write and socst."
codebook grade
list read write socst grade in 1/10
list read write socst grade in 1/10, nolabel

Let's label the data set itself so that we will remember what the data are.  We can also add some notes to the data set.

label data "High School and Beyond"

notes female:  the variable gender was renamed to female
notes race:  values of race coded as 5 were recoded to be missing
notes

There is another way to create variables in Stata that uses special functions.  Some of the functions available to you are listed in the table below.  Some examples of the use of the functions follow.

egen zread = std(read)
summarize zread
list read zread in 1/10

egen rmean = mean(read), by(ses)
list read ses rmean in 1/10

egen mread = median(read), by(prog)
list read prog mread in 1/10

save hs1

3.0 For more information

• Statistics with Stata 7
  Chapter 2 (especially pages 22-35)
• Stata Learning Modules
  Labeling Data
  Creating and recoding variables
• Stata Frequently Asked Questions
  How can I quickly convert many string variables into numeric variables?
  How can I quickly recode continuous variables into groups?
  How do I standardize variables in Stata?

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