The Stata Journal (2012) Vol. 12 No. 2, pp. 342-344.

In this Stata tip I discuss some uses of the fact that a model which constraints the effects of two or more variables to be equal is equivalent to a model that contains the sum of these variables. This fact makes it possible "add" categorical or ordinal variables or check whether the sum of various (Likert) items forms a reasonable scale.

- data set used in the examples
- do-file creating the example-data set from the 2010 General social Survey