Counting number of observations with same variables

[Alex Fernandez]

Hi
I have 100 observations in Stata, where each observations has 4 variables: student_id, school, level and class.

I want to generate a variable (Class_size), which tells me how many students belong to the same class, that is how many students_id share the same values of school, level and class. Is there a way to do this in Stata?

[Rich Goldstein]

yes, there is and if you follow the advice in the FAQ and post a data example, someone will give you exact code; in the meantime, you probably want to use an -egen- function; see

h egen

[Carlo Lazzaro]

Alex:
provided that Rich's wise recommendation still applies, do you mean something along the following lines?

. sysuse auto.dta
(1978 automobile data)

. egen wanted=group( rep78 foreign)
(5 missing values generated)

. bysort rep78 foreign: tab wanted

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 1, foreign = Domestic

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          2      100.00      100.00
------------+-----------------------------------
      Total |          2      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 2, foreign = Domestic

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          2 |          8      100.00      100.00
------------+-----------------------------------
      Total |          8      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 3, foreign = Domestic

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          3 |         27      100.00      100.00
------------+-----------------------------------
      Total |         27      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 3, foreign = Foreign

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          4 |          3      100.00      100.00
------------+-----------------------------------
      Total |          3      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 4, foreign = Domestic

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          5 |          9      100.00      100.00
------------+-----------------------------------
      Total |          9      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 4, foreign = Foreign

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          6 |          9      100.00      100.00
------------+-----------------------------------
      Total |          9      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 5, foreign = Domestic

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          7 |          2      100.00      100.00
------------+-----------------------------------
      Total |          2      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = 5, foreign = Foreign

group(rep78 |
   foreign) |      Freq.     Percent        Cum.
------------+-----------------------------------
          8 |          9      100.00      100.00
------------+-----------------------------------
      Total |          9      100.00

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = ., foreign = Domestic
no observations

----------------------------------------------------------------------------------------------------------------------------------------
-> rep78 = ., foreign = Foreign
no observations


. label define wanted 1 "rep78 = 1, foreign = Domestic" 2 "rep78 = 2, foreign = Domestic" 3 "rep78 = 3, foreign = Domestic"///
label define wanted 4 "rep78 = 3, foreign = Foreign" 5 "rep78 = 4, foreign = Domestic" 6 "rep78 = 4, foreign = Foreign" ///
7 "rep78 = 5, foreign = Domestic" 8 "rep78 = 5, foreign = Foreign", add

. label val wanted wanted

. tab wanted

         group(rep78 foreign) |      Freq.     Percent        Cum.
------------------------------+-----------------------------------
rep78 = 1, foreign = Domestic |          2        2.90        2.90
rep78 = 2, foreign = Domestic |          8       11.59       14.49
rep78 = 3, foreign = Domestic |         27       39.13       53.62
 rep78 = 3, foreign = Foreign |          3        4.35       57.97
rep78 = 4, foreign = Domestic |          9       13.04       71.01
 rep78 = 4, foreign = Foreign |          9       13.04       84.06
rep78 = 5, foreign = Domestic |          2        2.90       86.96
 rep78 = 5, foreign = Foreign |          9       13.04      100.00
------------------------------+-----------------------------------
                        Total |         69      100.00

.

[Nick Cox]

bysort school level class : gen class_size = _N