I did some assignments for what I believe was ACNielsen when I was in college. I'd get paid about €10 an hour to stand outside an Aldi store asking every customer that came out for their receipt. (Aldi also doesn't share its sales figures.) I'd also have to note every customer that didn't give their receipt. This gave ACNielsen information on the sales for that particular store. They, of course, did this at multiple locations and sold the gathered data to interested parties.