A garment retailer widened the aisles in one of its outlets, and sales at that outlet rose by 18 per cent over the following quarter. The head office concluded that wider aisles lead customers to buy more. Which of the following, if true, would most weaken that conclusion?

Hundreds of new people passing the door every working day would lift sales at that outlet whatever the aisles looked like, so the rise is explained without any effect of the widening. The customers' pleasant impression supports the head office rather than weakening it, the eight days of work would if anything have held sales back, and fewer items on display makes the rise harder to explain away, not easier.