2. Table 3 gives a contingency table of car accident counts cross-classified by

Question

2. Table 3 gives a contingency table of car accident counts cross-classified by three variables (1) Nature of injury (fatal or not). (2) Seat belt use (yes or no), and (3) Whether the occupant was ejected from the car (yes or no). The total number of accidents is considered fixed. Table 3: Accident counts by nature of injury, seat belt use and whether the occupant was ejected. injury eject counts 1,105 14 1 nonfatal yes yes yes yes 3 nonfatal no yes no yes nonfatal yes no yes no 2 a 4 fatal 5 6 fatal 7 nonfatal 8 fatal 483 4,624 497 no no 157,342 1,008 otal 576184 no no (a) (1 point) Estimate the difference in risk of a fatal accident between the seat belt and no seat belt groups. HINT: Estimate the conditional probabilities that the accident was fatal given seat belt status.

Explanation / Answer

Soliution

Probability of fatal accident with seat belts = (14+497)/(1105+14+4624+497) = 0.08189

Probability of fatal accident with no deat belts = (483+1008)/(41111+483+1457342+10008) = 0.00262

Risk difference = 0.08189-0.00262 = 0.07927

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