Scott Imberman wrote:
I was wondering if there is s a simple way to count the number of
unique values in a variable list for each observation. For example, i
am looking at the id numbers for colleges that a set of student attends.
A student may list having attended a college in multiple years like
this:
year id
1995 3
1996 2
1997 3
1998 3
I want to know how many unique schools the student attended (in this
example the answer is 2).
-------------------
In Stata terminology the information for the four years for one student
is not organized in one, but four observations (long format). A solution
follows here (but I guess there are simpler ways):
------------------------------------------------------------------------
clear
input student year school
17 1995 3
17 1996 2
17 1997 3
17 1998 3
18 1995 1
18 1996 1
18 1997 1
18 1998 1
19 1995 1
19 1996 2
19 1997 3
19 1998 4
end
gen x=1
sort student school
list
replace x=x[_n-1]+1 if student==student[_n-1] & school != school[_n-1]
replace x=x[_n-1] if student==student[_n-1] & school == school[_n-1]
by student: gen nschools=x[_N]
list
+----------------------------------------+
| student year school x nschools |
|----------------------------------------|
1. | 17 1996 2 1 2 |
2. | 17 1997 3 2 2 |
3. | 17 1995 3 2 2 |
4. | 17 1998 3 2 2 |
5. | 18 1997 1 1 1 |
|----------------------------------------|
6. | 18 1996 1 1 1 |
7. | 18 1995 1 1 1 |
8. | 18 1998 1 1 1 |
9. | 19 1995 1 1 4 |
10. | 19 1996 2 2 4 |
|----------------------------------------|
11. | 19 1997 3 3 4 |
12. | 19 1998 4 4 4 |
+----------------------------------------+
------------------------------------------------------------------------
Hope this helps,
Svend
________________________________________________________
Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6
DK-8000 Aarhus C, Denmark
Phone, work: +45 8942 6090
Phone, home: +45 8693 7796
Fax: +45 8613 1580
E-mail: [email protected]
_________________________________________________________
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