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 staffing agency On Assignment, Inc. (NYSE:ASGN) dropped as much as 19% today after it reported disappointing earnings.

So what: First-quarter revenue rose 5.7% to $430.0 million and net income was $39.1 million, or $0.75 per share. After taking out one-time items, earnings were $13.8 million, or $0.26 per share, which was well below Wall Street's expectations of $0.43 per share in earnings.  

Worse yet, management said second-quarter revenue would be between $454 million and $459 million, well short of the $469.6 million analysts were expecting.

Now what: The job market is certainly doing well, but staffing agency growth isn't what Wall Street had been expecting it would be. With shares trading at 23 times trailing earnings and growth slowing, I don't think there's a lot of reason to buy this dip, even today. I'd look for better value, like a P/E ratio in the low teens, or higher growth before jumping into shares of On Assignment.