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 P.F. Chang's (Nasdaq: PFCB) fell 13% in trading as investors threw their chopsticks down in disgust after the company released earnings.

So what: Revenue fell 1% to $311 million, but it was same-store sales falling 2.5% at P.F. Chang's restaurants and 2.7% at Pei Wei Diners that has investors concerned. Earnings per share fell to $0.40 from $0.55 a year ago, which is the same result analysts expected this quarter.

Now what: Same-store sales drive earnings, and even a small change can point to issues at a chain. This deterioration in sales has caused management to reduce earnings expectations from $2.15 to $2.20 per share down to $1.60 and $1.70 per share for the full year. I'm not seeing a big reason to buy today and would search for growing same store-sales like Chipotle (NYSE: CMG) if you're looking for a restaurant stock.

Interested in more info on P.F. Chang's? Add it to your watchlist.