The public's questions indicate a healthy dose of skepticism about polling. Their questions, however, are usually accompanied by a strong and sincere desire to find out what's going on under Gallup's hood. It turns out that the callers who reach Gallup's switchboard may be just the tip of the iceberg.
Survey researchers have actually conducted public opinion polls to find out how much confidence Americans have in polls -- and have discovered an interesting problem. People generally believe the results of polls, but they do not believe in the scientific principles on which polls are based. In a recent Gallup "poll on polls," respondents said that polls generally do a good job of forecasting elections and are accurate when measuring public opinion on other issues. Yet when asked about the scientific sampling foundation on which all polls are based, Americans were skeptical.
Most said that a survey of 1,, respondents -- a larger than average sample size for national polls -- cannot represent the views of all Americans. In addition to these questions about sampling validity, the public often asks questions about the questions themselves -- that is, who decides what questions to ask the public, and how those looking at poll results can be sure that the answers reflect the public's true opinion about the issues at hand.
Go here to jump to the section on computing the sample size. Probability sampling is the fundamental basis for all survey research. The basic principle: a randomly selected, small percent of a population of people can represent the attitudes, opinions, or projected behavior of all of the people, if the sample is selected correctly.
The fundamental goal of a survey is to come up with the same results that would have been obtained had every single member of a population been interviewed.
For national Gallup polls, in other words, the objective is to present the opinions of a sample of people which are exactly the same opinions that would have been obtained had it been possible to interview all adult Americans in the country.
The key to reaching this goal is a fundamental principle called equal probability of selection , which states that if every member of a population has an equal probability of being selected in a sample, then that sample will be representative of the population.
It's that straightforward. Thus, it is Gallup's goal in selecting samples to allow every adult American an equal chance of falling into the sample. How that is done, of course, is the key to the success or failure of the process. The first one thousand people streaming out of a Yankees game in the Bronx clearly aren't representative of all Americans. Now consider a group compiled by selecting 1, people coming out of a Major League Baseball game in every state in the continental United States -- 48, people!
We now have a much larger group -- but we are still no closer to representing the views of all Americans than we were in the Bronx. We have a lot of baseball fans, but, depending on the circumstances, these 48, people may not even be a good representative sample of all baseball fans in the country -- much less all Americans, baseball fans or not. When setting out to conduct a national opinion poll, the first thing Gallup does is select a place where all or most Americans are equally likely to be found.
That wouldn't be a shopping mall, or a grocery store, an office building, a hotel, or a baseball game. The place nearly all adult Americans are most likely to be found is in their home. So, reaching people at home is the starting place for almost all national surveys. By necessity, the earliest polls were conducted in-person, with Gallup interviewers fanning out across the country, knocking on Americans' doors.
This was the standard method of interviewing for nearly fifty years, from about to the mid s, and it was a demonstrably reliable method. Gallup polls across the twelve presidential elections held between and were highly accurate, with the average error in Gallup's final estimate of the election being less than 3 percentage points.
By , a sufficient proportion of American households had at least one telephone to make telephone interviewing a viable and substantially less expensive alternative to the in-person method. And by the end of the s the vast majority of Gallup's national surveys were being conducted by telephone. Gallup proceeds with several steps in putting together its poll with the objective of letting every American household, and every American adult have an equal chance of falling into the sample.
In the case of Gallup polls which track the election and the major political, social and economic questions of the day, the target audience is generally referred to as "national adults. In effect, it is the civilian, non-institutionalized population.
College students living on campus, armed forces personnel living on military bases, prisoners, hospital patients and others living in group institutions are not represented in Gallup's "sampling frame.
Although it would be a lot simpler if we used phone books to obtain all listed phone numbers in America and sampled from them much as you would if you simply took every 38th number from your local phone book , we would miss out on unlisted phone numbers, and introduce a possible bias into the sample.
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Search Search. This is a method pollsters commonly use to make a random selection within households without having to ask the respondent to provide a complete roster of adults living in the household. Gallup does not use the same respondent selection procedure when making calls to cell phones because they are typically associated with one individual rather than shared among several members of a household. When respondents to be interviewed are selected at random, every adult has an equal probability of falling into the sample.
Gallup's Daily tracking process now allows Gallup analysts to aggregate larger groups of interviews for more detailed subgroup analysis. But the accuracy of the estimates derived only marginally improves with larger sample sizes.
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