Stata tip 84: Summing missings

摘要

Here we have a grouping variable, g, and a response, y. When we collapse, by (g) to produce a reduced dataset of the sums of y, many users are surprised at the results. Clearly 1 + 2 + 3 + 4 = 10 and 5 + 6 + 7 = 18, so no surprises there, but the other results need more comment.