@sasman wrote: Thanks, that makes sense.   A followup question: say our data had age as a categorical variable (1 if someone is age 10-19, 2 if age is 20-29, etc). Is there a way I can use the value of the number of those with age=2 in computations, by writing something like "num_age[2]"? You might consider expanding on exactly what you are attempting, what some input data would look like and the desired output. That would be a significantly different post and you should ask a separate question with an appropriate title. The type of numeric computation and what the result could mean would go a long way to providing a cleaner solution.   If the original question has been adequately answered please mark the post gave the answer as the correct solution so others know the original question has been resolved. ... View more