a commonly accepted measure of market concentration. It is calculated as the sum of market shares squared and varies between 0 and 10,000 or 0 and 1, depending on whether percentages or percentage points are used.
note: Competition authorities use the HHI to assess the degree of market concentration. Different thresholds are used. A market scoring below 1,500 or 0.15 is considered competitive, between 1,500 and 2,500 or 0.15 and 0.25 is somewhat concentrated and above 2,500 or 0.25 highly concentrated. For commodities, the HHI is also used to assess the degree of concentration in producer countries, e.g. in the World Mining Database. Copper's HHI is below 1,000 and decreasing, meaning that copper's production is highly diversified and diversifying. Only gold does better among metals.