Seatbelts are more effective in saving lives than most previous studies suggest, while air bags are less effective, say Steven D. Levitt and Jack Porter in a new study published by the National Bureau of Economic Research.

Specifically, they found:

• Seatbelts reduce the likelihood of death in a crash by about 60 percent and air bags by 9 percent to 16 percent in front-end crashes.

• Using recent data on the cost of airbags and seatbelts, they estimate that the cost per life saved with seatbelts is about $30,000, compared with $1.6 million for airbags.

• Thus, for example, increasing seatbelt use by 10 percent would reduce fatalities more than by equipping all cars on the road with airbags.

Past studies of the effectiveness of airbags and seatbelts have been flawed because of sample selection bias since systematic data are collected only for fatal crashes -- thus excluding crashes in which seatbelts or airbags reduced injury severity so no one was killed.

Levitt and Porter only used data for cars involved in accidents in which someone in another vehicle was killed, which removes the bias in the sample selection, say the researchers. In other words, they didn't count people simply because they themselves were killed, but only when a person in the other car was killed; thus those who walked away relatively unscathed were as likely to be included in the sample as those who died.

Source: Gene Koretz, "Buckling Up Beats Air Bags," Business Week, September 6, 1999.