The US 500 Index (SPX) represents 500 large stocks listed on the NYSE or NASDAQ. The stocks are selected by the S&P specialists according to several criteria, such as; liquidity, size, and industry grouping.Reflecting the performance of the 500 large-cap companies from the major industries, the index is capitalisation-weighted and is often referred to as a benchmark for the U.S. stock market. The index value is updated every 15 seconds during the trading sessions on a daily basis.