I am trying to find a way to get the number of levels of a categorical variable as a single number. For example if I have a variable X with 4 levels I need to somehow get that number. If I type levelsof X I get the following 1 2 3 4 but I can't get only number 4 from there. Is there a way to do it using the levelsof or another command?
Various commands will give you the number of distinct values, for any kind of variable. ("Categorical variable" is a statistical concept, rather than a Stata concept.) Perhaps the simplest way to do it for one-off purposes is to ask for a one-way tabulation using tabulate. The number of distinct values is then the number of rows in that table, returned as r(r). Note that (1) you can suppress the table itself (which is useful in a program or do file) and (2) missing values are excluded by default:
. sysuse auto, clear
(1978 Automobile Data)
. qui tab foreign
. ret li
scalars:
r(N) = 74
r(r) = 2
. qui tab rep78
. ret li
scalars:
r(N) = 69
r(r) = 5
. qui tab rep78, missing
. ret li
scalars:
r(N) = 74
r(r) = 6
An extended review of this problem, pitched more generally, is available here. That paper introduces a distinct command. Its uses include direct support for looking at the number of distinct values systematically. search distinct in Stata to find a download source for the most recent version.
. distinct
| Observations
| total distinct
--------------+----------------------------
make | 74 74
price | 74 74
mpg | 74 21
rep78 | 69 5
headroom | 74 8
trunk | 74 18
weight | 74 64
length | 74 47
turn | 74 18
displacement | 74 31
gear_ratio | 74 36
foreign | 74 2