A polling firm wants to contact a random sample of people likely to vote in an upcoming nation-wide election. They will use a random digit dialer to generate and call phone numbers at random, so the poll will include people with landlines, unlisted numbers, and mobile phones. The random digit dialer skips invalid phone numbers. If a person doesn't answer a call, the dialer will try one more time, and then skip that number. When a person does answer, a pollster asks a set of initial questions -such as the person's age and whether or not they voted in previous elections to see if they are likely to vote in the upcoming election. If they are eligible and likely to vote, the pollster will then ask a series of questions about the election,​