Determine if values of one variable are all the same based on another variable

[Straso Jovanovski]

Hello, I want to determine if values of var1 are all the same based on a group of values (levels of an identifier variable) in var2 (both vars are strings).
I tried using:
bysort var2: egen byte differ = diff(var1)
however it does that comparison between variables and not with values within a variable, and also doesn't work with by option.
Thank you for any help.

[Rich Goldstein]

that will not work as your command is not legitimate (diff requires at least 2 variables in the parenthetical part and compares the values of those two variables); I think that what you want is more like:

bysort var2 (var1): gen byte differ=_n!=_N

[Friedrich Huebler]

I am not sure this is what Straso needs. Take the auto data and assume we want to compare the values of foreign for each value of rep78.

. sysuse auto, clear
. tab rep78 foreign

    Repair |
    Record |       Car type
      1978 |  Domestic    Foreign |     Total
-----------+----------------------+----------
         1 |         2          0 |         2 
         2 |         8          0 |         8 
         3 |        27          3 |        30 
         4 |         9          9 |        18 
         5 |         2          9 |        11 
-----------+----------------------+----------
     Total |        48         21 |        69

The way I understand the question, differ should take the value 1 if rep78 is 3, 4 or 5. With the code in post #2 we get:

. bysort rep78 (foreign): gen byte differ=_n!=_N
. list rep78 foreign differ if rep78==1

     +---------------------------+
     | rep78    foreign   differ |
     |---------------------------|
  1. |     1   Domestic        1 |
  2. |     1   Domestic        0 |
     +---------------------------+

. list rep78 foreign differ if rep78==5

     +---------------------------+
     | rep78    foreign   differ |
     |---------------------------|
 59. |     5   Domestic        1 |
 60. |     5   Domestic        1 |
 61. |     5    Foreign        1 |
 62. |     5    Foreign        1 |
 63. |     5    Foreign        1 |
     |---------------------------|
 64. |     5    Foreign        1 |
 65. |     5    Foreign        1 |
 66. |     5    Foreign        1 |
 67. |     5    Foreign        1 |
 68. |     5    Foreign        1 |
     |---------------------------|
 69. |     5    Foreign        0 |
     +---------------------------+

Instead, this FAQ may provide some answers to the original question: How do I count the number of distinct strings across a set of variables?

[Nick Cox]

http://www.stata.com/support/faqs/da...ions-in-group/ may also help.