Roger Partridge: The Rule of N - A short course in competitive arithmetic

In New Zealand economics, numbers have personalities. Two supermarkets are a duopoly. Three would be perfection – except four banks are still an oligopoly. One airline is intolerable, even though two always seem to collapse.

The equation is precise: current number (N) plus one equals salvation. This is the Rule of N – count the competitors, declare the number insufficient, and pronounce the market broken. It is beautifully simple, and completely backwards.