I’m using the monthly data and I’m trying to aggregate population totals by period. I’m using code that looks something like this:
collapse (sum) pop_start[iw=wtfinl], by(countyreal year month)
The problem is, I’m getting results that bounce all over the place. In any given county, the population from month to month will jump wildly up or down.
What am I doing wrong here? Might it be that I can only aggregate to the national level, and not the county level?
This is a graph of the average population by county over time: