S&P's price to 10-year average earnings (P/E10)

Historic Price-to-Earnings (P/E) ratio using reported earnings (as opposed to earnings estimates), that for "earnings" part can be found on Standard & Poor's website, where the latest earnings are posted on the earnings page.

The number we want is the sum of the reported earnings for the previous four quarters. Since the first quarter of 2009 earnings aren't available, we'll use the Q4 2008 earnings, which, subject to revision, is $14.97 per share (as of March 31). Thus the 2008 year-end P/E ratio for the S&P 500 is the December closing price of 903.25 divided by 14.97, which gives us the stunning P/E ratio of 60.3 — the highest in the history of the S&P Composite since 1871.

The average P/E over this timeframe is only 15. In fact, at the top of the Tech Bubble in 2000, the conventional P/E ratio was a mere 30. It peaked north of 47 two years after the market topped out.


If we calculate earnings based on Standard & Poor's earnings estimate for the first quarter (again, as of March 31), the number drops to $8.18. That gives us a P/E at yesterday's close of 102.

As these examples illustrate, in times of critical importance, the conventional P/E ratio often lags the index to the point of being useless as a value indicator. "Why the lag?" you may wonder. "How can the P/E be at a record high after the price has fallen so far?" The explanation is simple. Earnings fell far faster. In fact, Q4 earnings were negative — something that has never happened before in the history of the S&P Composite.

The P/E10 RatioLegendary economist and value investor Benjamin Graham noticed the same bizarre P/E behavior during the Roaring Twenties and subsequent market crash. Graham collaborated with David Dodd to devise a more accurate way to calculate the market's value, which they discussed in their 1934 classic book, Security Analysis. They attributed the illogical P/E ratios to temporary and sometimes extreme fluctuations in the business cycle. Their solution was to divide the price by the 10-year average of earnings, which we'll call the P/E10. In recent years, Yale professor Robert Shiller, the author of Irrational Exuberance, has reintroduced the P/E10 to a wider audience of investors. As the accompanying chart illustrates, this ratio closely tracks the real (inflation-adjusted) price of the S&P Composite.

With this method, the historic P/E average is 16.3, with a March monthly average P/E10 of 13.5 and a monthly close at P/E10 of 14.2. The ratio in this chart is doubly smoothed (10-year average of earnings and monthly averages of daily closing prices). Thus the fluctuations during the month aren't especially relevant (e.g., the difference between the March average and closing P/E10).

Of course, the historic P/E10 has never flat-lined on the average. On the contrary, over the long haul it swings dramatically between the over- and under-valued ranges. If we look at the major peaks and troughs in the P/E10, we see that the high during the Tech Bubble was the all-time high of 44 in December 1999. The 1929 high of 32 comes in at a distant second. The secular bottoms in 1921, 1932, 1942 and 1982 saw P/E10 ratios in the single digits.