S&P MidCap 400 Index

This Standard & Poor's index serves as a barometer for the U.S. mid-cap equities sector and is the most widely followed mid-cap index in existence. To be included in the index, a stock must have a total market capitalization that ranges from roughly $750 million to $3 billion dollars. Stocks in this index repsent household names from all major industries including energy, technology, healthcare, financial and manufacturing.

Taobiz explains S&P MidCap 400 Index

Like many other stock market indexes, the S&P 400 MidCap Index is a value-weighted index, meaning that the stocks with the largest market capitalization have the most significant impact on the movement of the index. Similarly, smaller movements in the smallest companies in the index have virtually no effect on the overall movement of the index. This is an important fact to remember for investors seeking persification, as market-cap weighted index funds primarily expose an investor to the movements of a small group of stocks, despite the broad name of the index itself.