How to create new variables

Sometimes, we wish to create variables that are functions of other variables. For example, we may need to calculate body mass index (BMI) using height and weight.

Let's begin by opening and describing an example dataset from the Stata website.

. use https://www.stata.com/users/youtube/rawdata.dta, clear
(Fictitious data based on the National Health and Nutrition Examination Survey)

. describe

Contains data from https://www.stata.com/users/youtube/rawdata.dta
 Observations:         1,268                  Fictitious data based on the
                                                National Health and Nutrition
                                                Examination Survey
    Variables:            10                  6 Jul 2016 11:17
                                              (_dta has notes)

 
Variable      Storage   Display    Value
    name         type    format    label      Variable label
 
id              str6    %9s                   Identification Number
age             byte    %9.0g                 
sex             byte    %9.0g                 Sex
race            str5    %9s                   Race
height          float   %9.0g                 height (cm)
weight          float   %9.0g                 weight (kg)
sbp             int     %9.0g                 Systolic blood pressure (mm/Hg)
dbp             int     %9.0g                 Diastolic blood pressure (mm/Hg)
chol            str3    %9s                   serum cholesterol (mg/dL)
dob             str18   %18s                  
 
Sorted by: id

The description tells us that the variable height is measured in centimeters (cm) and the variable weight is measured in kilograms (kg). We wish to calculate BMI, which is defined as weight in kilograms divided by the square of height measured in meters. Let's use Stata's generate command to create a new variable for height measured in meters. We simply divide height by 100 to convert centimeters to meters.

. generate heightm = height/100

Then we can create a variable for BMI using our new heightm variable.

. generate bmi = weight / heightm^2

Let's list the first five observations and summarize bmi to check our work.

. list weight height heightm bmi in 1/5

. summarize bmi


    Variable 
 
        Obs        Mean    Std. dev.       Min        Max
 
 
 
         bmi 
 
      1,268    25.77892    5.241681   15.43519   53.11815

Note that we could have divided height by 100 and created the bmi variable with one generate command.

. generate bmi2 = weight / (height/100)^2

. summarize bmi bmi2


    Variable 
 
        Obs        Mean    Std. dev.       Min        Max
 
 
 
         bmi 
 
      1,268    25.77892    5.241681   15.43519   53.11815
        bmi2 
 
      1,268    25.77892    5.241681   15.43518   53.11815

We also could have used Stata's replace command to replace bmi rather than generate a second bmi variable.

. replace bmi2 = weight / (height/100)^2
(0 real changes made)

. summarize bmi bmi2

    Variable 
 
        Obs        Mean    Std. dev.       Min        Max
 
 
 
         bmi 
 
      1,268    25.77892    5.241681   15.43519   53.11815
        bmi2 
 
      1,268    25.77892    5.241681   15.43518   53.11815

You can watch a demonstration of these commands by clicking on the link to the YouTube video below. You can read more about these commands by clicking on the links to the Stata manual entries below.

See it in action

Watch Data management: How to create a new variable that is calculated from other variables.