A chain of three eye clinics reports that the average time a patient waits before being seen fell from 50 minutes to 38 minutes over the year, and its managing partner concludes that the chain's clinics are now working faster than they were. Which of the following is an assumption on which the partner's conclusion depends?

The chain's average is the three clinics' waits weighted by how many patients each saw, so it can fall without a single clinic quickening if patients simply shift away from the slow one — a bus route altered, a clinic closed a day a week, a new road bringing patients to the fastest branch. The partner's conclusion therefore rests on the mix having stayed put, and denying that assumption dissolves his case. Each clinic seeing more patients is neither needed nor helpful, since the mix could still shift. No patient waiting above 38 minutes would make 38 an unusually high average rather than a mean. And equal opening days leave the distribution of patients across the three untouched.