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 professional staffing firm On Assignment (NYSE:ASGN) popped 11% today after its quarterly results and guidance topped Wall Street expectations.

So what: On Assignment's first-quarter results -- adjusted EPS of $0.35 on a revenue spike of 148% -- and current-quarter guidance were so strong, that analysts have no choice but to raise their valuation estimates, yet again. In fact, adjusted EBITDA expanded 50 basis points, to 8.8% from the year-ago period, fueling investor optimism over management's ability to sustain profitable growth.

Now what: Management now sees second-quarter EPS of $0.27-$0.29 on revenue of $410 million-$414 million, nicely ahead of Wall Street's view of $0.26 and $402 million. "We believe we are well positioned to meet or exceed our financial targets for the full year," President and CEO Peter Dameris said in a statement. "Demand for our services is strong and in most of our segments our growth is outpacing the market." With the stock hitting a new 52-week high today, and trading at a 25-plus P/E, however, much of that growth might already be baked into the valuation.

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Fool contributor Brian Pacampara has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.