HOW POLLS ARE CONDUCTED
Your Frequently Asked Questions Answered
By Frank Newport, Lydia Saad, David Moore
from Where America Stands, 1997 John Wiley & Sons, Inc.
Public opinion polls would have less value in a democracy if the public - the very
people whose views are represented by the polls - didn’t have confidence in the
results. This confidence does not come easily. The process of polling is often mysterious,
particularly to those who don’t see how the views of 1,000 people can represent those
of hundreds of millions. Many Americans contact the Gallup Organization each year
- To ask how our results can differ so much from their own, personal impressions of what
people think,
- To learn how we go about selecting people for inclusion in our polls, and
- To find out why they have never been interviewed.
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 1,500-2,000 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.
The Sampling Issue
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.
Selecting a Random Sample
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
1,000 people coming out of a Major League Baseball game in every state in the continental
United States -- 48,000 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 48,000 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 1935 to the mid 1980s, and it
was a demonstrably reliable method. Gallup polls across the twelve presidential elections
held between 1936 and 1984 were highly accurate, with the average error in Gallup’s
final estimate of the election being less than 3 percentage points.
By 1986, 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 1980s the vast majority of Gallup’s national
surveys were being conducted by telephone. Today, approximately 95% of all households have
a telephone and every survey reported in this book is based on interviews 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.
It’s important to go through this complicated procedure because estimates are that
about 30% of American residential phones are unlisted. 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.
The Number Of Interviews Or Sample Size Required
One key question faced by Gallup statisticians: how many interviews does it take to
provide an adequate cross-section of Americans? The answer is, not many -- that is, if the
respondents to be interviewed are selected entirely at random, giving every adult American
an equal probability of falling into the sample. The current US adult population in the
continental United States is 187 million. The typical sample size for a Gallup poll which
is designed to represent this general population is 1,000 national adults.
The actual number of people which need to be interviewed for a given sample is to some
degree less important than the soundness of the fundamental equal probability of selection
principle. In other words - although this is something many people find hard to believe -
if respondents are not selected randomly, we could have a poll with a million people and
still be significantly less likely to represent the views of all Americans than a much
smaller sample of just 1,000 people - if that sample is selected randomly.
To be sure, there is some gain in sampling accuracy which comes from increasing sample
sizes. Common sense - and sampling theory - tell us that a sample of 1,000 people probably
is going to be more accurate than a sample of 20. Surprisingly, however, once the survey
sample gets to a size of 500, 600, 700 or more, there are fewer and fewer accuracy gains
which come from increasing the sample size. Gallup and other major organizations use
sample sizes of between 1,000 and 1,500 because they provide a solid balance of accuracy
against the increased economic cost of larger and larger samples. If Gallup were to -
quite expensively - use a sample of 4,000 randomly selected adults each time it did its
poll, the increase in accuracy over and beyond a well-done sample of 1,000 would be
minimal, and generally speaking, would not justify the increase in cost.
Statisticians over the years have developed quite specific ways of measuring the
accuracy of samples - so long as the fundamental principle of equal probability of
selection is adhered to when the sample is drawn.
For example, with a sample size of 1,000 national adults, (derived using careful random
selection procedures), the results are highly likely to be accurate within a margin of
error of plus or minus three percentage points. Thus, if we find in a given poll that
President Clinton’s approval rating is 50%, the margin of error indicates that the
true rating is very likely to be between 53% and 47%. It is very unlikely to be higher or
lower than that.
To be more specific, the laws of probability say that if we were to conduct the same
survey 100 times, asking people in each survey to rate the job Bill Clinton is doing as
president, in 95 out of those 100 polls, we would find his rating to be between 47% and
53%. In only five of those surveys would we expect his rating to be higher or lower than
that due to chance error.
As discussed above, if we increase the sample size to 2,000 rather than 1,000 for a
Gallup poll, we would find that the results would be accurate within plus or minus 2% of
the underlying population value, a gain of 1% in terms of accuracy, but with a 100%
increase in the cost of conducting the survey. These are the cost value decisions which
Gallup and other survey organizations make when they decide on sample sizes for their
surveys.
The Interview Itself
Once the computer has selected a phone number for inclusion into a sample, Gallup goes
to extensive lengths to try to make contact with an adult American living in that
household. In many instances, there is no answer or the number is busy on the first call.
Instead of forgetting that number and going on to the next, Gallup typically stores the
number in the computer where it comes back up to be recalled a few hours later, and then
to be recalled again on subsequent nights of the survey period. This procedure corrects
for a possible bias which could occur in if we included interviews only with people who
answered the phone the first time we called their number. For example, people who are less
likely to be at home, such as young single adults, or people who spend a lot of time on
the phone, would have a lower probability of falling into the sample than an adult
American who was always at home and rarely talked on his or her phone. The call-back
procedure corrects for this possible bias.
Once the household has been reached, Gallup
attempts to assure that an individual within that household is selected randomly - for
those households which include more than one adult. There are several different procedures
that Gallup has used through the years for this within household selection process.
Gallup sometimes uses a shorthand method of asking for the adult with the latest birthday.
In other surveys, Gallup asks the individual who answers the phone to list all adults in
the home based on their age and gender, and Gallup selects randomly one of those adults to
be interviewed. If the randomly selected adult is not home, Gallup would tell the person
on the phone that they would need to call back and try to reach that individual at a later
point in time.
These procedures, while expensive and while not always possible in polls which are
conducted in very short time periods, help to ensure that every adult American has an
equal probability of falling into the sample.
The Questions
The technical aspects of data collection are critically important, and if done poorly,
can undermine the reliability of even a perfectly worded question. However, when it comes
to modern-day attitude surveys conducted by most of the major national polling
organizations, question wording is probably the greatest source of bias and error in the
data, followed by question order. Writing a clear, unbiased question takes great care and
discipline, as well as extensive knowledge about public opinion.
Even such a seemingly simple thing as asking Americans who they are going to vote for
in a forthcoming election can be dependent on how the question is framed. For example, in
a presidential race, the survey researcher can include the name of the vice presidential
candidates along with the presidential candidate, or can just mention the
presidential candidates’ names. One can remind respondents of the party affiliation
of each candidate when the question is read, or can mention the names of the
candidates without any indication of their party. Gallup’s rule in this situation is
to ask the question in a way which mimics the voting experience as much as possible. We
read the names of the presidential and vice presidential candidates, and mention the name
of the party line on which they are running. All of this is information the voter would
normally see when reading the ballot in the voting booth.
Questions about policy issues have an even greater range of wording options. Should we
describe programs like food stamps and Section 8 housing grants as "welfare" or
as "programs for the poor" when asking whether the public favors or opposes
them? Should we identify the Clinton health care bill as health care "reform" or
as "an overhaul of the health care system" when asking about congressional
approval of the plan? When measuring support for the US military presence in Bosnia should
we say the United States is "sending" troops or "contributing" troops
to the UN-sponsored mission there? Any of these wording choices could have a substantial
impact on the levels of support recorded in the poll.
For many of the public opinion areas covered in this book, Gallup is in the fortunate
position of having a historical track record. Gallup has been conducting public opinion
polls on public policy, presidential approval, approval of Congress, and key issues such
as the death penalty, abortion, and gun control for many years. This gives Gallup the
advantage of continuing a question in exactly the same way that it has been asked
historically, which in turn provides a very precise measurement of trends. If the exact
wording of a question is held constant from year to year, then substantial changes in how
the American public responds to that question usually represent an underlying change in
attitude.
For new questions which don’t have an exact analog in history, Gallup has to be
more creative. In many instances, even though the question is not exactly the same, Gallup
can follow the format that it has used for previous questions which have seemed to have
worked out as objective measures. For instance, when Gallup was formulating the questions
that it asked the public about the Persian Gulf War in 1990 and 1991, we were able to go
back to questions which were asked during the Vietnam War and borrow their basic
construction. Similarly, even though the issues and personalities change on the national
political scene, we can use the same formats which have been utilized for previous
presidents and political leaders to measure support for current leaders.
One of the oldest question wordings which Gallup has in its inventory is presidential job
approval. Since the days of Franklin Roosevelt, Gallup has been asking "Do you
approve or disapprove of the job (blank) is doing as president?" That wording has
stayed constant over the years, and provides a very reliable trend line for how Americans
are reacting to their presidents.
For brand new question areas, Gallup will often test several different wordings.
Additionally, it is not uncommon for Gallup to ask several different questions about a
content area of interest. Then in the analysis phase of a given survey, Gallup analysts
can make note of the way Americans respond to different question wordings, presenting a
more complete picture of the population’s underlying attitudes.
Through the years, Gallup has often used a split sample technique to measure the impact
of different question wordings. A randomly selected half of a given survey is administered
one wording of a question, while the other half is administered the other wording. This
allows Gallup to compare the impact of differences in wordings of questions, and often to
report out the results of both wordings, allowing those who are looking at the results of
the poll to see the impact of nuances in ways of addressing key issues.
Conducting the Interview
Most Gallup interviews are conducted by telephone from Gallup’s regional
interviewing centers around the country. Trained interviewers use computer assisted
telephone interviewing (CATI) technology which brings the survey questions up on a
computer monitor and allows questionnaires to be tailored to the specific responses given
by the individual being interviewed. (If you answer "yes, I like pizza," the
computer might be programmed to read "What is your favorite topping?" as the
next question.)
The interviews are tabulated continuously and automatically by the computers. For a
very short interview, such as Gallup conducted after the presidential debates in October
1996, the results can be made available immediately upon completion of the last interview.
In most polls, once interviewing has been completed, the data
are carefully checked and weighted before analysis begins. The weighting process is a
statistical procedure by which the sample is checked against known population parameters
to correct for any possible sampling biases on the basis of demographic variables such as
age, gender, race, education, or region of country.
Once the data have been weighted, the results are tabulated by computer programs which
not only show how the total sample responded to each question, but also break out the
sample by relevant variables. In Gallup’s presidential polling in 1996, for example,
the presidential vote question is looked at by political party, age, gender, race, region
of the country, religious affiliation and other variables.
Interpreting the Results
There are several standard caveats to observe when interpreting poll results. Primary
among these are issues discussed in this chapter: question wording, question order, the
sample population, the sample size, the random selection technique used in creating the
sampling frame, the execution of the sample (including the number calls backs and length
of the field period) and the method of interviewing (in person vs. telephone vs. mail).
Anyone using the Gallup Poll can do so with assurance that the data were obtained with
extremely careful and reliable sampling and interviewing methods. Gallup’s intent is
always to be fair and objective when writing questions and constructing questionnaires.
The original mission of polling was to amplify the voice of the public, not distort it,
and we continue to be inspired by that mission.
With those assurances in mind, the outside observer or researcher should dive into poll
data with a critical mind. Interpretation of survey research results is most importantly
dependent on context. What the American public may say about an issue is most
valuable when it can be compared to other current questions or to questions asked across
time. Where trend data exist, one should also look at changes over time and determine
whether these changes are significant and important. Let’s say, for example, that
Bill Clinton has a job approval rating of 48%. Is this a good rating or a poor rating? The
best way to tell is to look at history for context: compare it to Clinton’s ratings
throughout the rest of his presidency, then compare it to approval ratings for previous
presidents. Did previous presidents with this rating at the equivalent point in time tend
to get re-elected or not? Then it can be compared to approval ratings of Congress, of the
Republican and Democratic congressional leaders.
Gallup generally provides written analysis of our own polling data. But we also provide
ample opportunity for the press, other pollsters, students, professors and the general
public to draw their own conclusions about what the data mean. The results to all Gallup
surveys are in the "public domain" - once they have been publicly released by
us, anyone who chooses may pick up the information and write about it themselves. The
survey results are regularly published in the major media, in the Gallup Poll Monthly,
and on several electronic information services such as Nexus, the Roper Center and the
Internet. We also make the raw data available to researchers who want to perform more
complex statistical analysis. In addition to the exact question wordings and current
results, Gallup reports trend results to all questions that have been asked previously so
that even the casual observer can review the current results in context with public
opinion in the past.
The key concept to bear in mind when analyzing poll data is that public opinion on a
given topic cannot be understood by using only a single poll question asked a single time.
It is necessary to measure opinion along several different dimensions, to review attitudes
based on a variety of different wordings, to verify findings on the basis of multiple
askings, and to pay attention to changes in opinion over time.
This is good advice to bear in mind as you work your way through the topics presented
in Where America Stands, 1997.
Copyright© 1999 - The Gallup Organization
http://www.gallup.com/poll/faq.asp
