Science Fair Project Encyclopedia
Opinion polls are surveys of opinion using sampling. They are designed to represent the opinions of a population by asking a small number of people a series of questions and then extrapolating the answers to the larger group.
History of opinion polls
The first known example of an opinion poll was a local straw vote conducted by The Harrisburg Pennsylvanian in 1824, showing Andrew Jackson leading John Quincy Adams by 335 votes to 169 in the contest for the United States presidency. Such straw votes - unweighted and unscientific - gradually became more popular; but they remained local, usually city-wide phenomena. In 1916, the Literary Digest embarked on a national survey (partly as a circulation-raising exercise) and correctly predicted Woodrow Wilson's election as President. Mailing out millions of postcards and simply counting the returns, the Digest correctly called the following four presidential elections.
In 1936, however, the Digest came unstuck. Its 2.3 million "voters" constituted a huge sample; however they were generally more affluent Americans who tended to have Republican sympathies. The Literary Digest did nothing to correct this bias. The week before election day, it reported that Alf Landon was far more popular than Franklin D. Roosevelt. At the same time, George Gallup conducted a far smaller, but more scientifically-based survey, in which he polled a demographically representative sample. Gallup correctly predicted Roosevelt's landslide victory. The Literary Digest went out of business soon afterwards, while the polling industry started to take off.
Gallup launched a subsidiary in the United Kingdom, where it correctly predicted Labour's victory in the 1945 general election, in contrast with virtually all other commentators, who expected the Conservative Party , led by Winston Churchill, to win easily.
By the 1950s, polling had spread to most democracies. Nowadays they reach virtually every country, although in more autocratic societies they tend to avoid sensitive political topics. In Iraq, surveys conducted soon after the 2003 war helped to measure the true feelings of Iraqi citizens to Saddam Hussein, post-war conditions and the presence of US forces.
For many years, opinion polls were conducted mainly face-to-face, either in the street or in people's homes. This method remains widely used, but in some countries it has been overtaken by telephone polls, which can be conducted faster and more cheaply. In recent years, Internet surveys have become increasingly popular.
Potential for inaccuracy
Margin of error
All polls based on representative samples have a margin of error, which is a function of the number of people polled. The margin of error reflects the effects of chance in the sampling process, but does not reflect other sources of error, such as measurement error, errors in data processing, and non-representative samples (see below). A poll with a random sample of 1000 people has margin of sampling error of 3% for the estimated percentage of the whole population. A 3% margin of error means that 95% of the time the procedure used would give an estimate within 3% of the percentage to be estimated.
Since the margin of error differs with the percentage estimated, though, and margins of error in polls are usually estimated as the largest possible, the margin of error may not be a good guide to the accuracy of the specific percentage estimated.
However, since people asked to participate in a poll usually have the right to refuse, poll samples usually are not representative samples from a population. Because of this self-selection bias , the characteristics of those who agree to be interviewed may be markedly different from those who decline. There is also a possibility of error because some potential participants cannot be reached. In these cases, the risk of error may be larger than the purely statistical margin of error calculated from the size of the sample. Estimates based on polls are usually estimates of percentages of the population of people who take part in polls.
Wording of questions
It is well established that the wording of the questions, and the order in which they are asked, can influence results of polls. Thus comparisons between polls often boil down to the wording of the question. One way in which pollsters attempt to minimize this effect is to ask the same set of questions over time, in order to track changes in opinion. The most effective controls, used by attitude researchers, are:
- asking enough questions to allow all aspects of an issue to be covered and to control effects due to the form of the question (such as positive or negative wording), the adequacy of the number being established quantitatively with psychometric measures such as reliability coefficients, and
- analyzing the results with psychometric techniques which synthesize the answers into a few reliable scores and detect ineffective questions.
These controls are not widely used in the polling industry.
Another source of error is the use of nonrepresentative samples. For example, telephone sampling has a built-in error because in many times and places, those with telephones have generally been richer than those without. Alternately, in some places, many people have only mobile telephones. Because pollers cannot call mobile phones (due to technical reasons resulting from the way that telephone numbers for the poll are generated), these individuals will never be included in the polling sample. If the subset of the population without cell phones differs markedly from the rest of the population, these differences can skew the results of the poll. Polling organizations have developed many weighting techniques to help overcome these deficiencies, to varying degrees of success.
People asked to participate in opinion polls also have the right to refuse; this means that the sample is self-selected and consequently a non-probability sample.
Other major polling organizations in the United States include:
- Quinnipiac Polls , run by the University of Connecticut, and started as a student project.
- The Pew Charitable Trusts conducts polls concentrating on media and political beliefs.
- The Harris Poll .
- Nielsen Ratings, virtually always for television.
The best-known failure of opinion polling to date in the United States was the prediction that Thomas Dewey would defeat Harry S. Truman in the 1948 Presidential election. Major polling organizations, including Gallup and the Roper Center , indicated a landslide victory for Dewey.
In the United Kingdom, most polls failed to predict the Conservative election victories of 1970 and 1992, and Labour's victory in 1974. However, their figures at other elections have been generally good, although they have displayed a fairly consistent tendency throughout the 1990s and the early years of the 21st century to overstate Labour support and understate Conservative support.
The influence of opinion polls
By providing information about voting intentions, opinion polls can sometimes influence the behaviour of electors. This phenomenon is known as a Bandwagon effect when the poll prompts voters to back the candidate shown to be winning in the poll and as a Boomerang effect where the likely supporters of the candidate shown to be winning feel that s/he is "home and dry" and that their vote is not required, thus allowing another candidate to win. In the United Kingdom general election, 1997, then Cabinet Minister, Michael Portillo's constituency of Enfield was believed to be a safe seat but opinion polls showed the Labour candidate Stephen Twigg steadily gaining support which prompted undecided voters to support Twigg in order to remove Portillo. This is known as Tactical voting
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