Investing in index funds like the Vanguard S&P 500 ETF (VOO 0.66%) and the State Street SPDR S&P 500 ETF (SPY 0.68%) has proven to be one of the best strategies of all time.

These index funds give you exposure to the S&P 500 (^GSPC 0.74%), or 500 of the top publicly traded American companies across a range of sectors as curated by S&P Global, making them an easy way to get diversification and growth. The S&P has long had strict criteria for its membership to ensure that the index only contains the most qualified companies.

The requirements to be considered include:

• Minimum market cap: The actual number varies, but was recently around $15 billion. If a company falls below that level, they are at risk of being removed from the…