Chapter 3. Probability

76 Chapter 3: Probability1. Study is prospective since the data are collected directly during the conduct of the study by observing whathappens to the study participants after treatment.117 =7342. P(headache | Viagra)= 0. 1593. Compare P(headache | Viagra) with P(headache | placebo)29 =725P(headache | placebo)= 0. 040Risk of getting a headache is higher in Viagra treatment group than placebo group.4. Absolute risk reduction for headache in Viagra and placebo groupsAbsolute risk reduction = P(event occurring in treatment) − P(event occurring in control)=117734−29725= 0.1590 − 0.0400 = 0.1195. Number needed to treat to stop headacheFirst we find the absolute relative risk reduction for no headacheAbsolute risk reduction = P(event occurring in treatment) − P(event occurring in control)=617734−696725= 0.8406 − 0.9600 = 0.1191Number needed to treat = = 8.40.119(rounded up to 9)6. Odds in favor of headache and odds against headache for Viagra groupnumber with headache 117= =number with no headache 617number with no headache 617= = 5.number with headache 117Odds in favor of headache= 0. 190Odds against headache= 2747. Relative Risk of headache in treatment group compared with placebo group, interpret117P Proportion in treatment group 734 0.1594Relative Risk =t== = = 3.985PcProportion in control group 29 0.0400725The incidence or risk of getting a headache for the Viagra treatment group is almost four times as high as it isfor the placebo group.8. Odds ratio for headaches in treatment group compared with placebo group, interpretodds in favor of event for treatment (or exposed group)Odds ratio ==odds in favor of event for control group0.18960.0417= 4.54711729 617696The odds of getting a headache compared with not getting a headache are about four and a half times greater inthe Viagra than in the placebo group.=