An open-access poll is a type of opinion poll in which a nonprobability sample of participants self-select into participation. The term includes call-in, mail-in, and some online polls. The most common examples of open-access polls ask people to phone a number, click a voting option on a website, or return a coupon cut from a newspaper. By contrast, professional polling companies use a variety of techniques to attempt to ensure that the polls they conduct are representative, reliable and scientific. The most glaring difference between an open-access poll and a scientific poll is that scientific polls typically randomly select their samples and sometimes use statistical weights to make them representative of the target population. Advantages and disadvantages Since participants in an open-access poll are volunteers rather than a random sample, such polls represent the most interested individuals, just as in voting. In the case of political polls, such participants might be more likely voters. Because no sampling frame is used to draw the sample of participants, open-access polls may not have participants that represent the larger population. Indeed, they may be composed simply of individuals who happen to hear about the poll. As a consequence, the results of the poll cannot be generalized, but are only representative of the participants of the poll. One example of an error produced by an open access-poll was one taken by The Literary Digest to predict the 1936 United States presidential election. Similar polls by the magazine had correctly predicted the outcome of the four earlier presidential elections. The magazine's 1936 poll suggested that Alfred Landon would defeat Franklin D. Roosevelt by an overwhelming margin. In fact, the opposite happened. Later studies suggested that the main reason for the error was that Roosevelt's opponents were more vocal and thus more willing to respond to the magazine, compared to the silent majority who supported Roosevelt. By contrast, scientific opinion polls taken by George Gallup correctly showed a clear lead for Roosevelt, albeit still noticeably lower than what he achieved. A way to minimize that bias is to weigh the results in order to make them more representative of the overall population. This does not make the results of the poll completely representative of the population but it does help increase the chances of the results representing the overall population. Online poll An online poll is a survey in which participants communicate responses via the Internet,