A company requires all job applicants to undergo drug testing for employment. If the applicant has used drugs the test has a 90% chance of a positive result. If the applicant has not used drugs the test has an 85% chance of a negative result. 4% of all applicants have used drugs. If an applicant has a positive test, what is the probability he or she has actually used drugs?

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A company requires all job applicants to undergo drug testing for employment. If the applicant has used drugs the test has a 90% chance of a positive result. If the applicant has not used drugs the test has an 85% chance of a negative result. 4% of all applicants have used drugs. If an applicant has a positive test, what is the probability he or she has actually used drugs?

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Step 1: Introduction

The given probabilities are

P(positive result given drugs has been used)=0.90P(negative result given drugs has not been used)=0.85P(drugs has been used)=0.04P(drugs has not been used)=1-0.04                                                   =0.96

(as the sum of the probabilities are one)

The Bays' theorem is

P(Ai|B)=P(B|Ai)P(Ai)jP(B|Aj)P(Aj)

 

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