When groups differ in size, a plain average misleads. Weight each group by its size.
- Weighted average = sum of (value × weight) / sum of weights. Average price weights by units; average conversion weights by visitors.
- Never average averages: a 2% conversion rate on 9,000 visitors and 10% on 1,000 visitors isn't 6% overall.
- Mix effect: a total can move even when no part changes. If customers shift toward a low-margin product, the blended margin falls while every product's margin stays flat.
- The paradox case: prices can rise in every region while the average price falls, because more sales came from the cheapest region.
In an interview, say which effect you see: "the margin fell because of mix, not price."