You only find out who is swimming naked when the tide goes out, Warren Buffett wrote in a 2001 letter to Berkshire Hathaway (BRK.A) stockholders. That s wildly inaccurate beach advice. However, taken as financial advice, it s the sort of vague gem that always seems to apply to the crisis of the moment. It s timeless.

Recommendations to buy particular stocks don t last nearly as long. Investors who follow the advice of Wall Street analysts should always check the date. Dozens of studies of analyst recommendations published over the past two decades point to two broad findings. First, stocks with heaps of buy recommendations attached to them tend to perform no better than other stocks. Second, stocks with recent, positive changes in analyst opinions to buy from hold, for example tend to outperform the broad market over the following year. The difference is timing. Five buy recommendations issued a year ago hold less predictive power than one issued yesterday because factors like price/earnings ratios, sales growth rates and economic trends can change sharply in a year.

At least one analyst covering each of the three stocks below changed his or her published recommendation to buy (or outperform ) this week.

Alaska Air

Upgraded to Buy from Hold Sept. 22
Helane Becker, Jesup & Lamont

Alaska Air Group (ALK) flies passengers to almost as many cities in California as in Alaska (and has its corporate headquarters in Seattle). Its sales are forecast to decline 10% this year. That s not so bad when compared with giant airlines like American and United, whose corporate parents are expected to suffer 2009 sales declines of 17% and 21%, respectively. On Tuesday, Alaska Air lowered its third-quarter estimate for fuel costs and said some key sales measures (sales per passenger and per seat/mile) improved in August from July. A new baggage fee helped. Helane Becker of Jesup & Lamont upgraded the stock, citing valuation in a Tuesday note. Shares sell for less than 12 times the 2009 earnings consensus.

Gymboree

Upgraded to Outperform from Market Perform Sept. 22
Adrienne Tennant, FBR Capital Markets

Just over a year ago this column recommended shares of Gymboree as a safe haven in stormy markets. They re up 27%, vs. a 13% decline for the broad-market S&P 500 index. Once a play center and now a kids clothier, Gymboree is increasing its sales this year, unlike competitors The Children s Place and Gap. In a Tuesday investor note recommending the stock, Adrienne Tennant of FBR Capital Markets wrote that the company is gaining market share because of compelling products, that potential exists for sustained sales improvements at longstanding stores and that the stock is still cheap. It trades at 15 times earnings.

Dish Network

Upgraded to Outperform from Market Perform Sept. 22
Marci Ryvicker, Wells Fargo Securities

Dish Network offers satellite television service that competes with cable and with the satellite service of Direct TV. Because cable companies sell bundled television, telephone and Internet service, satellite companies must pair with telephone companies to compete. AT&T dumped Dish for DTV earlier this year, so although DTV is expected to increase its sales by 9% this year, Dish s sales are forecast to rise less than 1%. Perhaps that difference is more than reflected in the two stock prices, though. DTV sells for 19 times earnings and Dish just 10 times earnings. In a Tuesday upgrade note, Marci Ryvicker of Wells Fargo Securities wrote that Wall Street s expectations for the company are too low and that the share price should rise as the economy pulls out of recession.