While we could spend much time with probability, we will briefly review the main concepts of probability and then apply probability to our analysis of practical problems such as the chapter 4 problem on page 137. Table 4-1 is reproduced to the right.

Accurate drug testing is important. A drug test that frequently and incorrectly indicates the presence or absence of drugs is worse than useless. On the other hand, drug tests are seldom 100% correct.

Here is a standard method of estimating the probability with which tests will produce accurate results. A perfect drug test would correctly segregate the marijuana users from the non-users every time.

The results are summarized to the right.

Test 300 people for marijuana use.

Table 4-1	Person used marijuana	Person did not use marijuana
Positive Test (indicated presence of marijuana)	119 true positive	24 false positive
Negative Test (indicated absence of marijuana)	3 false negative	154 true negative
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