ANYONE WHO HAS flown in the past few months has noticed something new: For the most part, agents at airport security checks seem competent, efficient and friendly. That's an improvement brought to us by the new Transportation Security Administration. Now comes a touchier job for the TSA: measures, largely invisible to the public, to screen passengers. Civil liberties groups, the European Union and even parts of the Bush administration have complained that a proposed security system, Computer Assisted Passenger Prescreening System II, or CAPPS II, will unduly intrude on passenger privacy and convenience without much improving safety. Privacy diehards have organized a "Boycott Delta" Web site because the airline has agreed to help the TSA develop the system.

The TSA has set a high standard: It wants a system that could have caught hijack leader Mohamed Atta before Sept. 11, 2001. As envisioned, passengers would tell airline agents their name, address, phone number and birth date. That information would be checked against databases set up by the TSA; in five seconds the system would give the agent a number or color considered a safety rating, and no other information about how it was reached. An agent would not, as some people fear, know a passenger hadn't paid parking tickets or was a deadbeat dad. The wrong number or color would just subject you to additional bag checks.

The balance between efficacy and privacy depends on what databases the TSA uses. The FBI's terrorist watch list is already checked; if you're there, you land on a no-fly list and police are summoned to the airline counter. Other reviews the TSA is considering involve guesswork, but the best guesswork that can be done, such as flight patterns: Do you fly often to certain suspected countries? Do you tend to buy one-way tickets on short notice? The touchiest under consideration are credit ratings; given the ubiquity of identity theft and the high error rates in credit-rating reports, the TSA would have to come up with a pretty convincing reason for why that information would be relevant to a threat level.

The challenge for the TSA is focusing resources so agents don't waste time on obviously low-risk passengers. One approach is what Rafi Ron, an Israeli specialist consulting with Logan International Airport in Boston, calls highly trained intuition that Israeli airlines use. Agents act like police detectives; different ones ask the same questions several times and look for inconsistencies. Israeli agents flagged Richard Reid, the "shoe bomber," as highly suspicious and had the air marshal on his flight keep a constant eye on him. But Israelis deal with far fewer passengers. And to Americans, relying on intuition risks racial and ethnic profiling. Still the TSA ought to pursue screening methods that improve on the random checks used now. That should be doable without serious threats to privacy.