Speaking Stata: Is a variable constant?

Whether a variable is in fact constant|so that it takes on exactly the same 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 subsets of the dataset. It may arise because constancy is desirable or because constancy is undesirable, but either way we often need to check simply and quickly. In this column, I discuss several methods for checking. I do not claim to offer a complete analysis of why you might or should care but will give examples arising from experience.