The calculation of a performance index treats all cash dividends, interest payments or other income earned from the ownership of securities (e.g. proceeds from subscription rates for shares) as if they were reinvested in the fictitious portfolio. The index is likewise adjusted to take account of any changes in capital.
This means that the performance index is always higher than a price index. Performance indices often carry the designation "TR" (total return). The SPI (Swiss Performance Index) is the most widely-known performance index in Switzerland. The SMI is also calculated as a performance index although the media mainly refer to the level of the price index.