We should stress that a sample does not need to be completely random in order to be useful. It just needs to be random with respect to whatever we are asking about. Suppose we take an election poll based on only those voters whose names appear in the first ten pages of the phone book. This is a highly nonrandom sample of people. But unless having a name that begins with the letter A somehow correlates with political preference, our sample is random with respect to the question we are asking: How are you going to vote in the upcoming election?
