A city corporation posted four extra sweepers to each of the ten wards that had recorded the largest number of sanitation complaints in June. Over July the complaints from those ten wards fell by a third, and the corporation concludes that the extra sweepers brought them down. Which of the following, if true, most weakens the corporation's conclusion?

The ten wards were not picked at random; they were picked because June was an extreme month for them. A count that swings about a ward's own usual figure will, when it has just been at its highest, ordinarily be lower next month whatever anyone does, and five years of that pattern show the July fall was waiting to happen. How the sweepers were paid touches nothing. A slight rise elsewhere would, if anything, support the corporation rather than damage it. Where complaints are entered was the same in both months and cannot explain a change in either.