Here's an excerpt from John Hussman's Weekly Market Comment at HussmanFunds.com:
Present conditions create an urgency to examine all risk exposures. Once overvalued, overbought, overbullish extremes are joined by deterioration in market internals and trend-uniformity, one finds a narrow set comprising less than 5% of history that contains little but abrupt air-pockets, free-falls, and crashes.
In recent weeks, the market has transitioned to the most hostile return/risk profile we identify: the pairing of overvalued, overbought, overbullish conditions with deterioration in market internals and price cointegration – what we call “trend uniformity” – across a wide range of stocks, sectors, and security types (see my September 29, 2014 comment Ingredients of a Market Crash). As in 2007 and 2000, we’re observing characteristic features of that shift. One of those features is that early selling from overvalued bull market peaks tends to be indiscriminate, as deterioration in market internals and the “average stock” often precedes substantial losses in the major indices. As of Friday, only 28% of NYSE stocks are above their respective 200-day moving averages.