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 dialysis-services provider DaVita (NYSE: DVA) were reeling today, falling as much as 14% in intraday trading after getting slapped with a Wall Street downgrade.

So what: An analyst at Citigroup (NYSE: C) put a thumbs-down on DaVita today, dropping his rating from buy to hold. The primary concerns in the downgrade were the stock's valuation -- prior to today's drop it traded at 18 times trailing earnings -- and the company's exposure to legal problems with the government. Take the two together, and there's a case to be made for backing away from a bullish stance.

Now what: If Wall Street analysts were always the last word in what investors should do, investing would be a lot less confusing. We'd simply follow their recommendations and get on with more important things in our lives, like watching YouTube videos.

Unfortunately, it's not that simple since Wall Street analysts don't always get it right. DaVita bulls would be wise to take note of Citigroup's concerns and figure out whether they really are reason to throttle down the optimism, but knee-jerk selling is probably a bad idea.

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