There may be times that you would like to convert a continuous variable into groups. For example, you might want to convert a continuous reading score that ranges from 0 to 100 into 3 groups (say low, medium and high). You can use egen with the cut() function to do this quickly and easily, as illustrated below. We will illustrate this with the hsb2 data file with a variable called write that ranges from 31 to 67.
use https://stats.idre.ucla.edu/stat/stata/notes/hsb2, clear
summarize write
Variable | Obs Mean Std. Dev. Min Max
-------------+--------------------------------------------------------
write | 200 52.775 9.478586 31 67
We can use egen with the cut() function to make a variable called writecat that groups the variable write into the following 4 categories.
30 up to (but not including) 40 40 up to (but not including) 50 50 up to (but not including) 60 60 up to (but not including) 70
egen writecat = cut(write), at(30,40,50,60,70)
The tabstat command below is used to verify that the data are grouped as we expected. We can see that when writecat is in the lowest category (30) that write ranges from 31 to 39, and so forth as we expect, e.g., the values when writecat is in category 30 correspond to write having values of 30 up to (but not including) 40.
tabstat write, by(writecat) stats(min max)
Summary for variables: write
Group variable: writecat
writecat | Min Max
---------+--------------------
30 | 31 39
40 | 40 49
50 | 50 59
60 | 60 67
---------+--------------------
Total | 31 67
------------------------------
Here we use the same command but our last category is from 50 to 60. As you see, it generates a missing value because there are a number of values that are 60 or higher and thus outside of the range we specified. This shows that if there are values outside of the range you provide, those will be assigned a missing value.
egen writecat2 = cut(write), at(30,40,50,60) (53 missing value generated)
If we use the icodes option, cut() will create integer codes 0, 1, 2 and so forth. In the example below, you can see that it created codes 0, 1, 2 and 3.
egen writecat3 = cut(write), at(30,40,50,60,70) icodes
tabstat write, by(writecat3) stats(min max)
Summary for variables: write
Group variable: writecat3
writecat3 | Min Max
----------+--------------------
0 | 31 39
1 | 40 49
2 | 50 59
3 | 60 67
----------+--------------------
Total | 31 67
-------------------------------
If you use the label option (which automatically implies icode), then it will create integer values like above, but it will also create value labels. As you see below, the variable writecat4 is labeled 30- 40- 50- and 60-.
egen writecat4 = cut(write), at(30,40,50,60,70) label
tabstat write, by(writecat4) stats(min max)
Summary for variables: write
Group variable: writecat4
writecat4 | Min Max
----------+--------------------
30- | 31 39
40- | 40 49
50- | 50 59
60- | 60 67
----------+--------------------
Total | 31 67
-------------------------------
We use the nolabel option on the table command to suppress the display of the value labels and you can see that the variable really is coded 0, 1, 2 and 3.
tabulate writecat4, nolabel
writecat4 | Freq. Percent Cum.
------------+-----------------------------------
0 | 21 10.50 10.50
1 | 51 25.50 36.00
2 | 75 37.50 73.50
3 | 53 26.50 100.00
------------+-----------------------------------
Total | 200 100.00
If you prefer, you can ask cut() to choose the cutoffs to form groups with approximately the same number per group. Below we request the creation of 4 (roughly) equally sized groups.
egen writecat5 = cut(write), group(4) label
table write writecat5
--------------------------------------
writing | writecat5
score | 31- 45.5- 54- 60-
----------+---------------------------
31 | 4
33 | 4
35 | 2
36 | 2
37 | 3
38 | 1
39 | 5
40 | 3
41 | 10
42 | 2
43 | 1
44 | 12
45 | 1
46 | 9
47 | 2
49 | 11
50 | 2
52 | 15
53 | 1
54 | 17
55 | 3
57 | 12
59 | 25
60 | 4
61 | 4
62 | 18
63 | 4
65 | 16
67 | 7
--------------------------------------
For more information, see the help or reference manual about egen.