Over the past year or two, research analysts have caught plenty of grief for holding onto buy ratings for dear life. So what are they doing now? Why, they're changing their ratings more often than most people change underwear. OK, OK, not everyone. Just Merrill Lynch analyst David Risinger. And not on every stock. Just on Merck ( MRK), on which Risinger reinstated coverage last month. You see, as TheStreet.com pointed out Tuesday, Risinger this week changed his rating on the pharmaceutical conglomerate twice in one day. OK, OK. That's stretching it a little bit. Actually, he changed it twice in less than three hours, as far as we can tell. Our story starts innocently enough, back in Nov. 18, when Risinger launched coverage of Merck with a buy, citing his confidence in the company's 2003 targets and his belief that Merck would beat Wall Street's consensus estimates for earnings per share. Risinger puts a price target of $64 on Merck, which opened that morning at $55.88. Fast-forward a scant 10 trading days to this past Tuesday, when Merck announces it will hold a call to discuss earnings guidance for 2003 on Thursday, Dec. 5 -- only five days ahead of the company's annual analysts' day. So how does Risinger react to Tuesday's announcement of Thursday's forthcoming announcement? Why, he wavers. In a very big way. Citing Merck's year-earlier analysts' day, when lowered guidance apparently cast a pall over the stock, Risinger decided that if Merck was going out early with 2003 numbers, the numbers were going to be bad. "We are now concerned about EPS growth in 2003," Risinger wrote in a note that Briefing.com picked up at 11:30 a.m. EST. He cut his Merck rating down two notches, from buy to sell.