I need to group 3 variables to build an index in Stata. Each variable refers to a question in a questionnaire. The point is that they all have different scales: one variable has a scale from 1 to 7 (1 = never, 7 = always, another variable from 1=low to 3=high, and the third 1 to 2 (it's a yes or no question). How do I build an index with these three variables?