Reply #74: A little math (I think I did it right) [View All]

[Ivory_Tower]

I pulled all the base numbers out of thin air, so bear with me...

Assume a drug test is 98% accurate.

A company tests 10000 employees.
Assume 10% of the employees use drugs -- so 1000 employees use drugs, 9000 do not.
The test is 98% accurate. This means that the test will accurately test positive 98% of the time, and accurately test negative 98% of the time.

So, out of 1000 drug-using employees, 980 will be identified as drug users, 20 will not.
And out of 9000 drug-free employees, 8820 will be identified as drug free, and 180 will be identified as drug users.

This means the company will tag 980+180=1160 employees as drug-users, and 180/1160=15.5% of those will have been falsely accused. That is, the test we assume is 98% accurate in identifying drug users is far from that.