The term S&P 400 Mid Cap Index refers to a stock market index that measures the performance of mid-cap stocks traded in the United States equities markets. The S&P Mid Cap Index is maintained by S&P Dow Jones Indices.
Also referred to as the S&P 400, the S&P Mid Cap Index was first launched on June 19, 1991. The index provides investors with a measure of the performance of mid-sized companies in terms of market capitalization.
As the name implies, the index is made up of mid-sized companies. To be considered for inclusion, the company must have a market capitalization that ranges from $1.4 billion to $5.9 billion at the time of addition. However, over time the company's market cap will change and the actual market cap of equities in this index may range from a low of $900 million to over $10 billion. At the time of writing, the median market cap of the index was $3.5 billion.
Performance of the index is calculated based on market capitalization weights, which means the movement of common stocks of larger companies in the index, in terms of market capitalization, has a greater impact on the movement of the index. The index is float-weighted, meaning only the shares that are publicly traded are included in the weighting calculation. Finally, while foreign companies may be included in the index, the focus is American equities. The performance of the index can be tracked using the stock ticker MID.