Attempting to cash in on the capricious rise and fall of airfares is like trying to handicap a horse race or time the stock market: an exercise in frustration. But by analyzing historical pricing data, a new breed of travel Web sites promises to help consumers decide whether to click now or hold tight and hope for a better deal later.

Seattle-based newcomer www.Farecast.com uses fare data, seasonal trends and other factors to predict whether prices will rise, fall or hold steady on flights from 75 U.S. airports.

Tomorrow, the company will roll out a service that gives money back to some purchasers if Farecast's advice to wait because fares are dropping or staying the same turns out to be wrong.

"We're not clairvoyant," says CEO Hugh Crean, who says predictions have been accurate between 70 percent and 75 percent of the time. (A Seattle Times survey last fall pegged its accuracy rate closer to 61 percent.)

But by letting customers lock in the cheapest fare for one week, "we're putting our money where our mouth is."

How it works: You do a search between, say, Chicago and Miami for specific dates in March. If Farecast thinks the lowest fare of $300 will go down or hold steady, you can buy a Fare Guard (for $3 now, going up to $9.95 Feb. 1) that's good for a week and guarantees $300 is the most you will pay, assuming you're willing to take the cheapest flight offered. You'll get a daily e-mail that tracks fares for your trip. If the $300 fare drops, you saved money. If it rises, buy through Farecast, and it will pay the difference between the old and new lowest fares.

Another site, www.Farecompare.com, tackles the buy or hold question with a two-year history of fare changes for 77,000 North American and 200,000 international city pairs, rating the current best fare one to four stars. Four stars means the fare is at or near the historical low for that route.

Meanwhile, www.Hotwire.com just introduced TripStarter, a feature that shows two-year seasonal pricing trends for hotel rates in 80 destinations and for airfares in 5,000 U.S. and international city pairs.

Skeptics say the chaotic, complex nature of airfare pricing and inventory control means the sites will have a tough time beating airlines at their own game.

"There are so many variables. If a new airline enters a market, all bets are off," says Bob Cowen of www.InternetTravelTips.com. Using past data to shape current decisions "is like driving a car while looking in the rearview mirror."