TY  - JOUR
AU  - Levitt,Steven D.
AU  - Porter,Jack
TI  - Sample Selection in the Estimation of Air Bag and Seat Belt Effectiveness
JF  - National Bureau of Economic Research Working Paper Series
VL  - No. 7210
PY  - 1999
Y2  - July 1999
UR  - http://www.nber.org/papers/w7210
L1  - http://www.nber.org/papers/w7210.pdf
N1  - Author contact info:
Steven D. Levitt
Department of Economics
University of Chicago
1126 East 59th Street
Chicago, IL  60637
Tel: 773/834-1862
Fax: 773/702-8490
E-Mail: slevitt@midway.uchicago.edu
AB  - Measurement of seat belt and air bag effectiveness is complicated by the fact that systematic data are collected only for crashes in which a fatality occurs.  These data suffer from sample selection since seat belt and air bag usage influences survival rates which in turn determine whether a crash is included in the sample.  Past researchers either ignored sample selection or adopted indirect estimation methods subject to other important biases.  We propose a simple, but novel,  solution to the selection problem: limiting the sample to crashes in which someone in a different vehicle dies.  Under relatively weak conditions, consistent estimates can be obtained from this restricted sample.  Empirically, we find seat belts to be more effective in saving lives than most previous estimates.  Air bags, however, appear to be less effective than generally thought.  If our coefficients can be generalized to all crashes, the cost per life saved with seat belts is approximately $30,000, compared to $1.6 million for air bags.
ER  -