Here's a new update of our preferred market valuation method using the latest Standard & Poor's earnings estimates and the index price as of November 18.

The "price" part of the P/E calculation is available in real time on TV and the Internet. The "earnings" part, however, is more difficult to find. The authoritative source is the Standard & Poor's website, where the latest numbers are posted on the earnings page in a linked Excel file (see column D).

The table here shows the TTM earnings based on "as reported" earnings (Dec-08, Mar-09, and Jun-09). The values for the months between are linear interpolations from the quarterly numbers. The July through December numbers are based on Standard &P Poor's top-down estimate for September and December with interpolations for the months between.

The average P/E ratio since the 1870's has been about 15. What is it today? The November mid-month TTM P/E is 47.6. 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. So a P/E ratio in the high 40s with the index down approximately 29% from its October 2007 all-time high can't be taken seriously.

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 faster than price. In fact, the negative earnings of 2008 Q4 (-$23.25) is something that has never happened before in the history of the S&P 500. We are now recovering from this unprecedented situation. But a full recovery will take time.

The P/E10 Ratio
Legendary 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 real price by the 10-year average of real 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. The historic P/E10 average is 16.3. Here is an overlay chart of the P/E and P/E10 to facilitate comparison.

The Current P/E10
After dropping to 13.4 in March, the November 2009 interim-month P/E10 has rebounded to 20.4. The chart below gives us a historical context for these numbers. 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 monthly average and monthly close 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 44.2 in December 1999. The 1929 high of 32.5 comes in at a distant second. The secular bottoms in 1921, 1932, 1942 and 1982 saw P/E10 ratios in the single digits.

Where does the current valuation put us?
For a more precise view of how today's P/E10 relates to the past, our chart includes horizontal bands to divide the monthly valuations into quintiles — five groups, each with 20% of the total. Ratios in the top 20% suggest a highly overvalued market, the bottom 20% a highly undervalued market. What can we learn from this analysis? Over the past several months, the decline from the all-time P/E10 high dramatically accelerated toward value territory, with the ratio dropping from the 1st to the upper 4th quintile in March. The price rebound since March has now put the index near the top of the 2nd quintile — rather expensive!

A more cautionary observation is that every time the P/E10 has fallen from the first to the fourth quintile, it has ultimately declined to the fifth quintile and bottomed in single digits. Based on the latest 10-year earnings average, to reach a P/E10 in the high single digits would require an S&P 500 price decline below 600. Of course, a happier alternative would be for corporate earnings to make a strong and prolonged surge. When might we see the P/E10 bottom? These secular declines have ranged in length from over 19 years to as few as three. The current decline is now in its ninth year.