Resumen de Speaking Stata: s a variable constant?

Nicholas J. Cox

• Whether a variable is in fact constant—so that it takes on exactly thesame value in different observations—is of common concern in statistical work.The question may arise for all the observations in a dataset or for different subsetsof the dataset. It may arise because constancy is desirable or because constancyis undesirable, but either way we often need to check simply and quickly. In thiscolumn, I discuss several methods for checking. I do not claim to offer a completeanalysis of why you might or should care but will give examples arising fromexperience.