Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of mortgage insurer PMI Group (NYSE: PMI) spiked as much as 11% in intraday trading on heavier-than-average volume.

So what: The spike in PMI shares -- along with shares of fellow mortgage insurer Radian Group (NYSE: RDN) -- lines up almost perfectly with the big move in MBIA's (NYSE: MBI) shares today, suggesting that traders pushed up the mortgage insurers on the MBIA news. This, of course, makes little sense. The news out of MBIA today pertains specifically to the company's restructuring plans, and its ability to separate its public-finance insurance business from its structured-products insurance business. In other words, it's company-specific news that has little, if any, bearing on PMI or Radian.

Now what: Since there doesn't seem to be a solid reason for either of these stocks -- PMI or Radian -- to be gaining today, I'd be tempted to say that the gains won't hold. But that probably puts too much faith in the efficiency of the markets. The one thing that I can say for sure is that these have been particularly volatile stocks, and they will continue to be volatile as the companies work through the tough housing market and try to regain a solid footing.

Want to keep up to date on these stocks? Add PMI Group to your watchlist. Add Radian Group to your watchlist.


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Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.