The SMIM (SMI Mid) includes the Swiss equity markets 30 largest mid-cap stocks that are not already represented in the blue chip SMI index. Like all other SIX Swiss Exchange AG share indices, it is free-float capital weighted and only the tradable shares of a given company are used in calculating the index. As is the case with the SMI, the selection of its components is made according to the market capitalisation and trading turnover of the given stock, whereby the SPI EXTRA serves as the basis for the SMIM.
The SMIM is calculated both as a performance and a price index. It was standardised as at 31 December 1999 with a baseline value of 1000 for the performance index and 100 points for the price index; it was formally introduced on 15 November 2004. To enhance tradability and comparability with other indices, the price index was pegged at 1000 on 2 August 2005, retroactive to 31.12.1999.