The price-to-earnings ratio (P/E) is one of the most widely used tools by which investors and analysts determine a stock's relative valuation. The P/E ratio helps one determine whether a stock is overvalued or undervalued. A company's P/E can also be benchmarked against other stocks in the same industry or against the broader market, such as the S&P 500 Index.
Sometimes, analysts are interested in long-term valuation trends and consider the P/E 10 or P/E 30 measures, which average the past 10 or past 30 years of earnings, respectively. These measures are often used when trying to gauge the overall value of a stock index, such as the S&P 500, because these longer-term measures can compensate for changes in the business cycle.
The P/E ratio of the S&P 500 has fluctuated from a low of around 5x (in 1917) to over 120x (in 2009 right before the financial crisis). The long-term average P/E for the S&P 500 is around 16x, meaning that the stocks that make up the index collectively command a premium 16 times greater than their weighted average earnings.