Correction Appended

THE Web has given airline customers more convenience and more power. The ability to compare prices instantly at several airlines — something that was previously available only to travel agents — can't help but keep prices down.

But the airlines are still in control. The complicated algorithms they employ to analyze demand, competitors' prices and other data are the reason the same flight costs $350 one day and $550 the next. Here, online travel sites like Expedia and Travelocity aren't much help. That's because "Expedia's real customers are the travel companies — not you," writes John Battelle of Searchblog (battellemedia.com).

Farecast, a new Web service still being tested, monitors and analyzes price data and gives probabilities on when and by how much future fares might rise or fall. The "when" is crucial. Fares tend to fluctuate, but the trick is to know when they will hit their low point. Farecast is designed to predict it for you.

For Mr. Battelle, Farecast represents a potential return to the Web's early promise of shifting power to consumers. That was thwarted, he writes, when merchants began to collude with one another and with aggregators. "If you think AutoByTel or Expedia is on your side, you're kidding yourself."