Region A: £3.9m over 6 stores. Region B: £2.5m over 14 stores. What were the higher region's sales per store?

Convert each series to the same per-unit rate (value ÷ count) before comparing — never compare raw totals. The correct answer is £650k. Traps to avoid: £279k comes from the "cross divide" error; £417k comes from the "cross divide" error; £320k comes from the "pooled" error; £179k comes from the "absolute not per store" error.