Users of drug-tests need to be aware that no drug-tests are perfect.  There will always be people who do not use drugs but are identified as drug users.  Conversely, there will always be individuals who do use drugs but are misidentified as non-users.
Following are data from an observational study conducted on drug and non-drug users.
Sample                                                 Tested Positive (as drug users)  Tested Negative (as non drug users)
Out of 50 real drug users,             44 tested positive by test,            6 tested negative by test
Out of 950 non-drug users,          22 tested positive by test,            928 tested negative by test
What is P(real non-drug user and test positive)?