Chapter 3. Probability

Chapter 3: Probability 779. Should Viagra users be concerned about headache as an adverse reaction?There is some basis for being concerned since headaches occur much more often for the Viagra treatment groupthan for the placebo group.Facial Injuries and Bicycle Helmets. In Exercises 10-19, use the data in the accompanying table (based on datafrom “A Case-Control Study of the Effectiveness of Bicycle Safety Helmets in Preventing Facial Injury,” byThompson, Rivara, and Wolf, American Journal of Public Health, Vol. 80, No. 12).Helmet Worn No Helmet TotalFacial Injuries Received 30 182 212All Injuries nonfacial 83 236 319Total 113 418 53110. The study is retrospective since data are collected for events that have occurred in the past and are based onarchival data.number with facial injuries in no helmet groupnumber in no helmet group182= =41811. P(facial injuries | no helmet)= 0. 435No, this probability should not be used as an estimate of incidence since subjects were not selected at randomfrom the population of all bicycle riders.12. Is risk ratio justified for these data?No, since in a retrospective study incidence rates are based on the definitions used by the researcher incategorizing the data that are collected based on past events rather than being directly observed.13. Absolute Risk Reduction for helmet group and no helmet groupAbsoluterisk reduction = P(event occurringin treatment) − P(event occurringin control)=30113−182418= 0.2654 − 0.4354 = 0.17014. Odds in favor of facial injury for no helmet groupnumber with facial injurynumber with no facial injury182236Odds in favor of facial injury= = = 0. 77115. Odds ratio for facial injuries for no helmet group compared with helmet group, interpret182oddsin favor of event for no helmet group0.7712Odds ratio ==2.13430 236 = =oddsin favor of event for helmet group0.361483The odds of receiving a facial injury compared with not receiving a facial injury are 2.134 times higher for theno helmet group than for the helmet group.