New Panda data refresh rolling out today: 1.2% of English queries affected. Background: goo.gl/8Zqy1
— A Googler (@google) January 22, 2013
The link leads to Amit Singhal’s “More Guidance on Building Quality Sites” on the Google Webmaster Central blog. If you haven’t read the article, do. It explains exactly what Google is looking for.
If you’re not following the rules, look to be affected every time Panda rolls out.
And even if you are, these filter runs can still affect you. See if there’s much change in your site traffic over the next couple of days. If there is, you’re going to want to look further into the cause, but it may be due to this update.