There can only be one winner in an election race, and political polls are judged by their accuracy in forecasting who that will be. This is daunting indeed in the case of primary elections to fill local offices, when only a small fraction of the electorate may turn out. Turnout is affected by the weather and by public interest in concurring local or statewide contests. It makes a very big difference in some presidential election years but not in others. Calculations of turnout are based on the number of valid ballots cast in relation to the voting-age population, though this includes millions of aliens, recent movers and felons who cannot be counted exactly.
Some states allow registration at voting time.
An unknown number of enthusiasts register in more than one precinct. Between 70 and 80 percent of election poll respondents claim to be registered, but not all of them are. Many registered voters don t vote, and there is no easy way of determining who will. Today most survey findings are published with a brief accompanying statement about their margin of error, commonly given as plus or minus three percentage points when the response is evenly divided in a sample of 1,000. The margin gets smaller as the sample increases and as the response gets farther from an even division.