1. The problem statement, all variables and given/known data All athletes at the Olympic games are tested for performance-enhancing steroid drug use. The imperfect test gives positive results (indicating drug use) for 90% of all steroid-users but also (and incorrectly) for 2% of those who do not use steroids. Suppose that 5% of all registered athletes use steroids. If an athlete is tested negative, what is the probability that he/she uses steroids? 2. Relevant equations 3. The attempt at a solution I did P(defective)=.05 P(Tested defective|defective)=1 P(Tested defective|good)=.02 From there I did P(TD)=(1*.05)+(.02*(1-.05))=.069 (1*.05)/.069=.724 which was wrong. Am I missing a step?