With reference to the information provided in Exercises 10.54, 10.55, and 10.57, there was one more question of interest to the researchers. If people in cars with air bags are more likely to be wearing seat belts, then how much of the improvement in fatality rates with air bags is really due to seat belt usage? The harmful- event fatalities were then classified ­according to both availability of air bags and seat belt usage. The data are given here.

a. Use the information in the previous table and the data from Exercise 10.57 to compute the fatality rates for the four air bag and seat belt combinations.
b. Describe the confounding effect of seat belt usage on the effect of air bags on reducing fatalities.

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Seat Belt Usage No Seat Belt Air Bags Yes No Total Seat Belt Total 8,626 2374 16,000 10,650 20550 31.200 19.276 27924 47200