The S&P 500 is market-capitalization-weighted index, which means the stocks that have a higher market capitalization will be weighted higher in the index. If a company in the index with a high market capitalization starts performing especially well or poorly, the change will impact the index more than if a company with a smaller market capitalization soars or tumbles. This means that the value of the S&P 500 is always changing, depending on how the stocks included are doing throughout the day. The average annual returns of the index vary, but the annualized total return is around 10% since the index's inception, according to the S&P Dow Jones Indices.