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 hospital operator Health Management Associates (NYSE: HMA) were looking seriously ill today, as they fell as much as 31% after the company's general counsel abruptly resigned.

So what: The resignation marred what could have otherwise been a cheery day for HMA investors. The company released updated guidance for its fourth quarter and the full year for 2011. For the quarter, the bottom line looks like it will be much better than expected, with earnings per share of $0.25 to $0.26 against analysts' expectations of $0.18. Meanwhile, the company set an anticipated EPS range for 2012 of $0.80 to $0.90. Analysts had been looking for $0.81.

However, the resignation of general counsel Tim Parry couldn't be overlooked. Currently, HMA is embroiled in a lawsuit involving Medicare billing and a former employee-turned-whistleblower. That the company's chief of legal would resign while that is ongoing is certainly a worrisome sign.

Now what: "Innocent until proven guilty" is an important aspect of our legal system. For investors concerned that HMA might eventually be found guilty, though, the time to sell is before the rest of the market rushes for the exits.

So what should investors be doing right now? First, they should sit down to assess the probability that there was wrongdoing at HMA -- an outcome that seems more likely after Parry's resignation. Along with that, they should try to determine what the actual impact would be on the company if wrongdoing is proved. Finally, they can use those estimates to figure out whether today's share price doesn't yet reflect the potential broadsiding that the stock could take on the downside or if Mr. Market has gotten overly dour, overestimated the impact, and has created an opportunity in HMA's shares.

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