The key is being able to identify which is the most consistent interrelationship and when we have a divergence in that particular relationship attempt to glean future market direction.
With stocks down, the long end of the Treasury yield curve sold off hard today. That seldom happens and its clear to me the Treasury market is telling us something about stocks.
With the S&P 500 down .39% today, TLT (iShares Barclays 20+ Year Bond ETF) was down over twice as much (.79%). Here's a weekly chart of TLT:
Fundamentally, the market is spooked not so much about China as the media hyped all day but by concern about whether Credit Default Swaps will be triggered on Greek debt later this week. The nervousness was manifest by the VIX (CBOE Volatility Index) edging up and the CBOE Equity Only Put/Call ratio jumping. And yet, despite these concerns Treasuries were abandoned as a "safe haven" trade.