Stata tip 108: On adding and constraining

Maarten L. Buis

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.

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