Probability and statistics allow us to make decisions when dealing with uncertain variables. For example, assume you are the network administrator of a large firm and have ordered 100 RAID (redundant array of independent disks) devices. You would like to make sure all of the RAIDs work before sending them to different departments. But you do not have time to check all of the devices. If you check some of them and show they work, how confident could you be that all 100 devices work?

Of course, testing all of the RAIDs would be the best way to be confident, but assume you do not have the time. If you only have time to check 5 of the 100, and none of the 5 are defective, how confident would you be that none of the other microprocessors are defective (that is, all 100 devices are fully functional)?

Assume there are exactly 5 defective RAIDs out of the 100 RAID devices. What is the probability of testing 20 RAIDs at random and finding all of them nondefective?