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Stata FAQ
How can I randomly assign observations to groups in Stata?

The trick here is to create a random variable, sort the dataset by that random variable, and then assign the observations to the groups. Let's use the hsb2 dataset as an example by randomly assigning 50 observations to each of four groups.
use http://www.ats.ucla.edu/stat/stata/notes/hsb2, clear

generate rannum = uniform()
sort rannum
generate grp = .
replace grp = 1 in 1/50
replace grp = 2 in 51/100
replace grp = 3 in 101/150
replace grp = 4 in 151/200

tabulate grp

        grp |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         50       25.00       25.00
          2 |         50       25.00       50.00
          3 |         50       25.00       75.00
          4 |         50       25.00      100.00
------------+-----------------------------------
      Total |        200      100.00
      
sort id
list id grp in 1/20

            id        grp 
  1.         1          2  
  2.         2          3  
  3.         3          2  
  4.         4          3  
  5.         5          2  
  6.         6          3  
  7.         7          1  
  8.         8          2  
  9.         9          2  
 10.        10          4  
 11.        11          1  
 12.        12          1  
 13.        13          4  
 14.        14          2  
 15.        15          2  
 16.        16          1  
 17.        17          3  
 18.        18          2  
 19.        19          3  
 20.        20          1
Of course, when you try this the grp number for each id will be in a different pattern because we are using a random process to assign observations to groups.

For more information

See the Stata manual or Stata Help for functions.

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