The chances that Gallup will contact any one person to participate in one of its national telephone polls are extremely small. Even though Gallup conducts countless surveys in the context of national public opinion polling -- more than 350,000 each year -- that's a small fraction of the estimated more than 105 million U.S. households.
Still, Gallup accurately represents the views of the U.S. adult population because it has a system in place to randomly select survey participants. The precise methods Gallup uses are designed to give every household in America an equal chance of being selected for every survey. This ensures that Gallup has a random cross section of the population that represents adults of every gender, race, religion, region, party affiliation, etc. in the country and in the correct proportions.
Essentially, even though Gallup may not include you in the survey, your views are represented by other Americans sharing your views who are included survey.
Furthermore, the laws of probability statistics define the accuracy expected when applying random selection techniques to Gallup's surveys. These are the same laws that define the likelihood you will win any game of chance such as a lottery or raffle. Given this statistical foundation to Gallup's selection methods means the result of any given poll of 1,000 national adults has a maximum margin of error of ±4 percentage points. That means that if Gallup conducted the same poll 100 times at the same exact time the result to any question on that survey would come within four percentage points of the "true" figure in 95 of those surveys.
A classic demonstration of this involves setting up a bowl with an equal number of well-mixed red and blue marbles -- say 1,000 each. If you reach in and blindly and select 100 marbles, you are highly likely to get close to 50 marbles of each color. If you do the same experiment 100 times and plot the number of blue marbles you get each time on a graph, the resulting pattern will be a bell-shaped curve with the "correct" answer of 50 being the most common result. The majority of the other results (95%) will fall in a close range around 50. When Gallup conducts its national telephone polls, picture the company's telephone interviewing centers reaching out and randomly selecting households in the same way. There is a lot more complexity to America than red and blue marbles; Gallup is reaching into a "bowl" of numbers representing a vast variety of people and attitudes. But with Gallup's random selection methods, the national surveys reflect all of these in their appropriate proportions.
Read more in Gallup's longer paper: "How are polls conducted?"