Caltech Science Exchange / Topics / Voting and Elections / Polling

How Do Political Polls Work?

Political polls provide valuable but often limited insights into to the preferences and beliefs of certain groups of people, often voters in a specific country, state, or district. Polling results can inform policy, track the likely outcomes of elections, and help candidates and government officials decide where to invest time, campaign funds, and other resources. Furthermore, some experts argue that the ability of independent organizations to conduct and report the results of polls, free from government interference, is emblematic of a free and fair society.

Because posing questions to every member of a community would be costly and logistically difficult, pollsters instead survey a smaller, but representative subset of a population, known as a sample. However, even the best polls are subject to error, and response rates are declining, further complicating accuracy. A high-quality sample includes all types of people that make up the population. While larger samples are generally more precise, a sample size of just 1,000 to 1,500 people can be enough to estimate national opinion in the United States—though this precision often diminishes in closer elections or smaller geographic areas.

One method used to build a representative sample is random sampling, which gives everyone in the population an equal chance of being included or not. But in practice, this is difficult to achieve. Pollsters typically draw a random sample of participants from computer-generated lists of landline and cell phone numbers (Increasingly, polls are also being conducted online). However, this alone does not ensure a random sample because some groups of people are likelier than others to have a phone or participate in polls. To avoid overrepresenting some groups and underrepresenting others, factors such as age, race, gender, educational attainment, or region are given more or less weight in the final polling results so the sample more closely matches the demographics of the full population.

How do you know if a poll is trustworthy? Credible polls report both their margin of error and sampling methods. The margin of error indicates how much poll results could vary from reality but doesn't account for other errors, such as poor question design or response biases. Other factors to consider are the poll's funders, the pollster's sampling methods, and whether the questions were neutrally worded. It is also important to remember that national polls may not accurately reflect smaller subgroups or individual states, which are often crucial in elections. Polling firms who participate in industry transparency initiatives have been found to be more accurate than those that do not.