Over time, a number of theories and mechanisms have been offered to explain erroneous polling results. Some of these reflect errors on the part of the pollsters; many of them are statistical in nature. Some blame respondents for not providing genuine answers to pollsters, a phenomenon known as
social desirability-bias (also referred to as
the Bradley effect or
the Shy Tory Factor); these terms can be quite controversial.
Margin of error due to sampling Polls based on samples of populations are subject to
sampling error which reflects the effects of chance and uncertainty in the sampling process. Sampling polls rely on the
law of large numbers to measure the opinions of the whole population based only on a subset, and for this purpose the absolute size of the sample is important, but the percentage of the whole population is not important (unless it happens to be close to the sample size). The possible difference between the sample and whole population is often expressed as a
margin of error – usually defined as the radius of a 95% confidence interval for a particular statistic. One example is the percent of people who prefer product A versus product B. When a single, global margin of error is reported for a survey, it refers to the maximum margin of error for all reported percentages using the full sample from the survey. If the statistic is a percentage, this maximum margin of error can be calculated as the radius of the confidence interval for a reported percentage of 50%. For a poll with a random sample of 1,000 people reporting a proportion around 50% for some question, the sampling margin of error is approximately ±3% for the estimated proportion of the whole population. A 3% margin of error means that if the same procedure is used a large number of times, 95% of the time the true population average will be within the sample estimate plus or minus 3%. The margin of error can be reduced by using a larger sample, however if a pollster wishes to reduce the margin of error to 1% they would need a sample of around 10,000 people. In practice, pollsters need to balance the cost of a large sample against the reduction in sampling error and a sample size of around 500–1,000 is a typical compromise for political polls. (To get complete responses it may be necessary to include thousands of additional participators.) Another way to reduce the margin of error is to rely on
poll averages. This makes the assumption that the procedure is similar enough between many different polls and uses the sample size of each poll to create a polling average. Another source of error stems from faulty demographic models by pollsters who weigh their samples by particular variables such as party identification in an election. For example, if you assume that the breakdown of the US population by party identification has not changed since the previous presidential election, you may underestimate a victory or a defeat of a particular party candidate that saw a surge or decline in its party registration relative to the previous presidential election cycle. Sample Techniques are also used and recommended to reduce sample errors and errors of margin. In chapter four of author Herb Asher he says,"it is probability sampling and statistical theory that enable one to determine sampling error, confidence levels, and the like and to generalize from the results of the sample to the broader population from which it was selected. Other factors also come into play in making a survey scientific. One must select a sample of sufficient size. If the sampling error is too large or the level of confidence too low, it will be difficult to make reasonably precise statements about characteristics of the population of interest to the pollster. A scientific poll not only will have a sufficiently large sample, it will also be sensitive to response rates. Very low response rates will raise questions about how representative and accurate the results are. Are there systematic differences between those who participated in the survey and those who, for whatever reason, did not participate? Sampling methods, sample size, and response rates will all be discussed in this chapter" (Asher 2017). A caution is that an estimate of a trend is subject to a larger error than an estimate of a level. This is because if one estimates the change, the difference between two numbers
X and
Y, then one has to contend with errors in both
X and
Y. A rough guide is that if the change in measurement falls outside the margin of error it is worth attention.
Nonresponse bias Since some people do not answer calls from strangers, or refuse to answer the poll, poll samples may not be representative samples from a population due to a
non-response bias. Response rates have been declining, and are down to about 10% in recent years. Various pollsters have attributed this to an increased skepticism and lack of interest in polling. Because of this
selection bias, the characteristics of those who agree to be interviewed may be markedly different from those who decline. That is, the actual sample is a biased version of the universe the pollster wants to analyze. In these cases, bias introduces new errors, one way or the other, that are in addition to errors caused by sample size. Error due to bias does not become smaller with larger sample sizes, because taking a larger sample size simply repeats the same mistake on a larger scale. If the people who refuse to answer, or are never reached, have the same characteristics as the people who do answer, then the final results should be unbiased. If the people who do not answer have different opinions then there is bias in the results. In terms of election polls, studies suggest that bias effects are small, but each polling firm has its own techniques for adjusting weights to minimize selection bias.
Response bias Survey results may be affected by
response bias, where the answers given by respondents do not reflect their true beliefs. This may be deliberately engineered by unscrupulous pollsters in order to generate a certain result or please their clients, but more often is a result of the detailed wording or ordering of questions (see below). Respondents may deliberately try to manipulate the outcome of a poll by e.g. advocating a more extreme position than they actually hold in order to boost their side of the argument or give rapid and ill-considered answers in order to hasten the end of their questioning. Respondents may also feel under social pressure not to give an unpopular answer. For example, respondents might be unwilling to admit to unpopular attitudes like
racism or
sexism, and thus polls might not reflect the true incidence of these attitudes in the population. In American political parlance, this phenomenon is often referred to as the
Bradley effect. If the results of surveys are widely publicized this effect may be magnified – a phenomenon commonly referred to as the
spiral of silence. Use of the
plurality voting system (select only one candidate) in a poll puts an unintentional bias into the poll, since people who favor more than one candidate cannot indicate this. The fact that they must choose only one candidate biases the poll, causing it to favor the candidate most different from the others while it disfavors candidates who are similar to other candidates. The
plurality voting system also biases elections in the same way. Some people responding may not understand the words being used, but may wish to avoid the embarrassment of admitting this, or the poll mechanism may not allow clarification, so they may make an arbitrary choice. Some percentage of people also answer whimsically or out of annoyance at being polled. This results in perhaps 4% of Americans reporting they have personally been
decapitated.
Wording of questions Among the factors that impact the results of opinion polls are the wording and order of the questions being posed by the surveyor. Questions that intentionally affect a respondents answer are referred to as
leading questions. Individuals and/or groups use these types of questions in surveys to elicit responses favorable to their interests. For instance, the public is more likely to indicate support for a person who is described by the surveyor as one of the "leading candidates". This description is "leading" as it indicates a subtle bias for that candidate, since it implies that the others in the race are not serious contenders. Additionally, leading questions often contain, or lack, certain facts that can sway a respondent's answer. Argumentative Questions can also impact the outcome of a survey. These types of questions, depending on their nature, either positive or negative, influence respondents' answers to reflect the tone of the question(s) and generate a certain response or reaction, rather than gauge sentiment in an unbiased manner. In opinion polling, there are also "
loaded questions", otherwise known as "
trick questions". This type of leading question may concern an uncomfortable or controversial issue, and/or automatically assume the subject of the question is related to the respondent(s) or that they are knowledgeable about it. Likewise, the questions are then worded in a way that limit the possible answers, typically to yes or no. Another type of question that can produce inaccurate results are "
Double-Negative Questions". These are more often the result of human error, rather than intentional manipulation. One such example is a survey done in 1992 by the
Roper Organization, concerning the
Holocaust. The question read "Does it seem possible or impossible to you that the
Nazi extermination of the Jews never happened?" The confusing wording of this question led to inaccurate results which indicated that 22 percent of respondents believed it seemed possible the Holocaust might not have ever happened. When the question was reworded, significantly fewer respondents (only 1 percent) expressed that same sentiment. Thus comparisons between polls often boil down to the wording of the question. On some issues, question wording can result in quite pronounced differences between surveys. This can also, however, be a result of legitimately conflicted feelings or evolving attitudes, rather than a poorly constructed survey. A common technique to control for this bias is to rotate the order in which questions are asked. Many pollsters also split-sample. This involves having two different versions of a question, with each version presented to half the respondents. 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.. However, as it is important that questions to test the product have a high quality, survey methodologists work on methods to test them. Empirical tests provide insight into the quality of the questionnaire, some may be more complex than others. For instance, testing a questionnaire can be done by: • conducting
cognitive interviewing. By asking a sample of potential-respondents about their interpretation of the questions and use of the questionnaire, a researcher can • carrying out a small pretest of the questionnaire, using a small subset of target respondents. Results can inform a researcher of errors such as missing questions, or logical and procedural errors. • estimating the measurement quality of the questions. This can be done for instance using test-retest, quasi-simplex, or mutlitrait-multimethod models. • predicting the measurement quality of the question. This can be done using the software Survey Quality Predictor (SQP).
Involuntary facades and false correlations One of the criticisms of opinion polls is that societal assumptions that opinions between which there is no logical link are "correlated attitudes" can push people with one opinion into a group that forces them to pretend to have a supposedly linked but actually unrelated opinion. That, in turn, may cause people who have the first opinion to claim on polls that they have the second opinion without having it, causing opinion polls to become part of
self-fulfilling prophecy problems. It has been suggested that attempts to counteract unethical opinions by condemning supposedly linked opinions may favor the groups that promote the actually unethical opinions by forcing people with supposedly linked opinions into them by ostracism elsewhere in society making such efforts counterproductive, that not being sent between groups that assume ulterior motives from each other and not being allowed to express consistent critical thought anywhere may create psychological stress because humans are sapient, and that discussion spaces free from assumptions of ulterior motives behind specific opinions should be created. In this context, rejection of the assumption that opinion polls show actual links between opinions is considered important.
Coverage bias Another source of error is the use of samples that are not representative of the population as a consequence of the methodology used, as was the experience of
The Literary Digest in 1936. 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. In some places many people have only
mobile telephones. Because pollsters cannot use automated dialing machines to call mobile phones in the United States (because the phone's owner may be charged for taking a call), these individuals are typically excluded from polling samples. There is concern that, 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, with varying degrees of success. Studies of mobile phone users by the Pew Research Center in the US, in 2007, concluded that "cell-only respondents are different from landline respondents in important ways, (but) they were neither numerous enough nor different enough on the questions we examined to produce a significant change in overall general population survey estimates when included with the landline samples and weighted according to US Census parameters on basic demographic characteristics." This issue was first identified in 2004, but came to prominence only during the 2008
US presidential election. In previous elections, the proportion of the general population using cell phones was small, but as this proportion has increased, there is concern that polling only landlines is no longer representative of the general population. In 2003, only 2.9% of households were wireless (cellphones only), compared to 12.8% in 2006. This results in "
coverage error". Many polling organisations select their sample by dialling random telephone numbers; however, in 2008, there was a clear tendency for polls which included mobile phones in their samples to show a much larger lead for
Obama, than polls that did not. The potential sources of bias are: • Some households use cellphones only and have no landline. This tends to include minorities and younger voters; and occurs more frequently in metropolitan areas. Men are more likely to be cellphone-only compared to women. • Some people may not be contactable by landline from Monday to Friday and may be contactable only by cellphone. • Some people use their landlines only to access the Internet, and answer calls only to their cellphones. Some polling companies have attempted to get around that problem by including a "cellphone supplement". There are a number of problems with including cellphones in a telephone poll: • It is difficult to get co-operation from cellphone users, because in many parts of the US, users are charged for both outgoing and incoming calls. That means that pollsters have had to offer financial compensation to gain co-operation. • US federal law prohibits the use of automated dialling devices to call cellphones (
Telephone Consumer Protection Act of 1991). Numbers therefore have to be dialled by hand, which is more time-consuming and expensive for pollsters. == Failures ==