from Consuelo Mack's March 20/09 WealthTrack show Merrill Lynch's chief investment strategist, Richard Bernstein "RICHARD BERNSTEIN: You know the S&P has reported earnings for the S&P 500 on a reported basis and even on an operating basis are negative for the first time. In other words, at a loss. In other words, the companies in aggregate are producing a loss for the first time ever in history. This has never happened before. It's our forecast that this may happen again in the first quarter coming up, and if that happens, it's entirely possible that on a trailing 12 month basis, the earnings for the S&P may be negative which would mean for the first time ever in history, you won't be able to calculate a price earning ratio for the S&P." http://www.wealthtrack.com/transcript_03-20-2009.php
The problem with looking at the S&P aggregate earnings is that they calculate them by simply adding up the earnings of all of the companies on the index, while the index itself is weighted by market cap. So, a company that has a tiny weight on the index could produce a large loss, and skew the aggregate earnings by quite a lot. I've read opinions on calculating it differently, and there doesn't seem to be a good solution to calculating it fairly. Here's a website where all of the companies on the S&P are listed with their weight on the S&P. http://www.indexarb.com/indexComponentWtsSP500.html