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 weight management company Weight Watchers International (NASDAQ:WW) plummeted 18% today, after its full-year outlook disappointed Wall Street.

So what: Weight Watchers' fourth-quarter results -- adjusted EPS of $0.96 on revenue of $407.9 million -- managed to top estimates, but downbeat guidance for 2013 reinforced concerns over rapidly declining demand. Management cited thinning attendance at its diet meetings, poor recruitment numbers, and higher expenses for the gloomy outlook, giving investors plenty of negative vibes over its competitive position going forward.

Now what: Management now expects full-year 2013 EPS of $3.50-$4.00, well below Wall Street's view of $4.74 per share. "Our current marketing has not been as effective in this tough economic and increasingly competitive environment," CEO David Kirchhoff said . "In this context, we are taking appropriate steps to address these near-term challenges while continuing to pursue our long-term growth strategies." With the stock flirting with its 52-week low, once again, and trading at a P/E of 10, betting on that turnaround might even pay off over time.

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