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.

The market sell-off continued Monday, and with 30 minutes left in trading the Dow Jones Industrial Average (DJINDICES:^DJI) was down 302 points, or 1.93%. The only real negative data point today is the Institute for Supply Management PMI falling from 56.5% in December to 51.3% last month. But that was enough to give investors reason to sell stocks, and 29 of 30 Dow components are in the red today.

The Dow's one bright spot
The only Dow component moving higher is Pfizer (NYSE:PFE), which is up 1% on news that its breast cancer treatment palbociclib significantly increased patient survival rates in a midstage trial.  

Analysts estimate the drug could peak at $5 billion in annual sales, so this could be a huge opportunity for Pfizer, especially with the Food and Drug Administration giving palbociclib breakthrough status, which will speed up the approval process.

Is this time to panic?
Outside of that lone bright spot, the market is in full freak-out mode. So, should you be selling along with the market today?

It's important to put sell-offs like this in perspective. The Dow has still returned 13% over the past year and earnings season has been solid almost across the board. As of Jan. 31, 74% of the 251 S&P 500 member companies to issue earnings numbers had beaten earnings estimates and 67% had beaten sales estimates. That's no longer enough to get investors to buy stocks; instead they're picking apart any single negative in earnings reports or guidance.

I think what we're seeing is a hangover from last year's market performance, which was driven by multiple expansion, not earnings growth. Expectations got too high and investors were just looking for an excuse to sell-off.

Don't panic over the recent market moves, because fundamentals are still strong. The U.S. economy is growing, earnings are increasing, and unemployment is slowly falling. Those factors will drive the market over the long term, and this sell-off will be a good buying opportunity for investors who can look past the short-term market volatility to find great values.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.