[U] User's Guide

[ U ] 13.7 Explicit subscripting 155Here is what we will see when we type these commands:. use http://www.stata-press.com/data/r11/gxmpl7, clear. by region, sort: generate avgx=sum(x)/_n. by region: replace avgx=avgx[_N](46 real changes made)In our example, there are no missing observations on x. If there had been, we would have obtainedthe wrong answer. When we created the running average, we typed. by region, sort: generate avgx=sum(x)/_nThe problem is not with the sum() function. When sum() encounters a missing, it adds zero tothe sum. The problem is with n. Let’s assume that the second observation in the first region hasrecorded a missing for x. When Stata processes the third observation in that region, it will calculatethe sum of two elements (remember that one is missing) and then divide the sum by 3 when it shouldbe divided by 2. There is an easy solution:. by region: generate avgx=sum(x)/sum(x