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Why do I get the message “outcome does not vary” when I perform a logistic or logit regression?

Title   Interpreting “outcome does not vary” message when running logistic
Author Paul Lin, StataCorp
Date November 1996

Answer

You probably have a dependent variable that is coded 1 and 2 for the two outcomes

        1       meaning the event did not occur
        2       meaning the event did occur

Stata wants the coding

        0       meaning the event did not occur
        1       meaning the event did occur

All you have to do to fix the problem is

        . replace dep_var = dep_var - 1

Details

The coding

        0       meaning the event did not occur
        1       meaning the event did occur

is just an example of the type of coding Stata requires. Stata actually assumes the dependent variable is of the form

         0      meaning the event did not occur
      nonzero   meaning the event did occur 

and draws no distinction between the nonzero numbers. Thus, if you had a dependent variable that recorded the number of times an event occurred and you now wanted to analyze simply whether the event occured 1 or more times, you could use that count variable as a dependent variable with logistic, logit, or probit.

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