Hi Family,
I have 1 categorical variable that has 3 grps( blue,yellow, green), I wanted check if the variance between only these two groups( blue and green) differs. So how do i use if condition to constraint the categorical variable to work with only the 2 groups out of the 3.

Given that;
the value label for blue=1 yellow=2 and green=3
wait= Weight of individuals

Below is the command I used but stata still generate an error.


sdtest wait , by( grps ) if grps==1 & grps==3

option if not allowedr(198);

I need your help?

Thank you.
Amidu