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Abstract
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Using the National Automotive Sampling System (NASS) Crashworthiness Data System (CDS) we have looked at the effectiveness of airbags on the probability of dying in an accident, controlling for other factors. We found that airbags are actually associated with increased probability of death in accidents. The situations in which airbags are most dangerous are low-speed collisions when the occupants are not wearing seatbelts.
How can we reconcile our findings with previous studies that found airbags to be saving lives? For example, the study by Crandall, Olson, and Skiar (2001) is similar to ours, in that a logistic regression model is used to estimate odds ratios and the same effects are controlled for in the model. They looked at head-on collisions in passenger cars, and found the odds ratio for deployment of the airbags is 0.71, with 95 percent CI (.58, .87). Their study used a different NHTSA database called the Fatality Analysis Reporting System (FARS).
When we look at the random sample of all accidents (CDS), we get that airbags are associated with increased risk of death, and this increase is due mostly to more deaths with airbags in low-speed crashes and no seatbelts. However, if we limit the dataset to include only collisions in which a fatality occurred, we get a significantly reduced risk of death due to airbags. This is analogous to doing a comparison with the same numerators but different denominators—it is not unlikely that we will get different results.
Here is a more dramatic analogy: If you look at people who have some types of cancer, you will see that those who get radiation treatment have a better chance of surviving than those who don’t. However, radiation is inherently dangerous, and could actually cause cancer. If you give everyone radiation treatments whether they have cancer or not, you will probably find an increased risk of death in the general population. Making everyone have airbags and then verifying the effectiveness using only fatal crashes is like making everyone have radiation and then estimating lives saved by looking only at people who have cancer. Overall, there will be more deaths if everyone is given radiation, but in the cancer subset, radiation will be effective.
Using the CDS dataset for the analysis, we find that airbags are associated with significantly more deaths, rather than fewer. Using a subset of the CDS data, we can mimic previous analysis and reproduce the results. Because of this, we are confident that our analyses better reflect the actual effectiveness of airbags in the general population. The evidence shows that airbags do more harm than good. |