Gallup World Poll Methodology
To ensure that the Gallup World Poll survey data are representative of 95% of the world's adult population, the following methodology is employed:
- The target population is the entire civilian, non-institutionalized, aged 15 and older population.
- With some exceptions, all samples are probability based and nationally representative.*
- There is a standard set of core questions used around the world.
- In some regions, supplemental questions are asked in addition to core questions. For example, the questions used in heavily indebted poor countries are tailored toward providing information about progress on the Millennium Development Goals.
- The questionnaire is translated into the major languages of each country.**
- Interviewing supervisors and interviews are trained not only on the questionnaire, but also on the execution of field procedures. This interviewing training usually takes place in a central location.
- Telephone surveys are used in countries where telephone coverage represents at least 80% of the population or is the customary survey methodology. In countries where telephone interviewing is employed, Random-Digit-Dial (RDD) or a nationally representative list of phone numbers is used. Telephone methodology is typical in the United States, Canada, Western Europe, Japan, Australia, etc.
- In the developing world, including much of Latin America, the former Soviet Union countries, nearly all of Asia, the Middle East, and Africa, an area frame design is used for face-to-face interviewing.
- Face-to-face interviews are approximately 1 hour, while telephone interviews are about 30 minutes.
- Quality control procedures are used to validate that correct samples are selected and that the correct person is randomly selected in each household.
- The typical World Poll survey includes at least 1,000 surveys of individuals. In some countries, oversamples are collected in major cities or areas of special interest. Additionally, in some large countries, such as China and Russia, sample sizes of at least 2,000 are collected. Although rare, in some instances the sample size is between 500 and 1,000.
STEP 1 -- Selecting Primary Sampling Units (PSUs): In countries where face-to-face surveys are conducted, the first stage of sampling is the identification of PSUs (Primary Sampling Units), consisting of clusters of households. PSUs are stratified by population size and or geography and clustering is achieved through one or more stages of sampling. Where population information is available, sample selection is based on probabilities proportional to population size, otherwise simple random sampling is used.
STEP 2 -- Selecting Households: Random route procedures are used to select sampled households. Unless an outright refusal occurs, interviewers make up to three attempts to survey the sampled household. To increase the probability of contact and completion, attempts are made at different times of the day, and where possible on different days. If an interview cannot be obtained at the initial sampled household, a simple substitution method is used.
In countries where telephone interviewing is employed, Random-Digit-Dial (RDD) or a nationally representative list of phone numbers is used. In select countries where cell phone penetration is high a dual sampling frame is used. At least three attempts are made to reach a person in each household.
STEP 3 -- Selecting Respondents: In face-to-face and telephone methodologies, random respondent selection is achieved by using either the latest birthday or Kish grid method.
These probability surveys are valid within a statistical margin of error, also called a 95% confidence interval. This means that if the survey is conducted 100 times using the exact same procedures, the margin of error would include the "true value" in 95 out of the 100 surveys. With a sample size of 1,000, the margin of error for a percentage at 50% is ±3 percentage points.***
Because these surveys use a clustered sample design, the margin of error varies by question, and if a user is making critical decisions based on the margin of error, he or she should consider inflating the margin of error by the design effect. The design effect accounts for the potential of correlated responses and increase in the margin of error caused by the sample of clusters of households in PSU.
Contact Dr. Robert D. Tortora, Chief Methodologist of Gallup, at firstname.lastname@example.org for more detailed information or any questions regarding the methods used in the Gallup World Poll.
*Exceptions include areas where the safety of the interviewing staff is threatened, scarcely populated islands in some countries, and areas that interviewers can reach only by foot, animal, or small boat.
**The translation process starts with an English, French, or Spanish version, depending on the region. A translator who is proficient in the original and target languages translates the survey into the target language. A second translator reviews the language version against the original version and recommends refinements.
***Assuming other sources of error, such as nonresponse, by some members of the targeted sample are equal. Other errors that can affect survey validity include measurement error associated with the questionnaire, such as translation issues and coverage error, where a part or parts of the target population, aged 15 and older, have a zero probability of being selected for the survey.
The Gallup World Poll global indexes span multiple economic, political, and social topics that correlate with real-world outcomes.
Law and Order Index
The Law and Order Index measures security levels that respondents report for themselves and their families. It incorporates two questions that gauge respondents' sense of personal security and two questions that specifically address the incidence of crime.
Food and Shelter Index
The Food and Shelter Index measures whether a respondent has experienced deprivation in the areas of food or shelter. Two items that ask about respondents' ability to afford food or shelter in the past year compose this index.
Personal Economy Index
The Personal Economy Index measures respondents' personal economic situations and the economics of the community where they live.
The Personal Health Index measures perceptions of one's own health and incidence of pain, sadness, and worry.
Citizen Engagement Index
The Citizen Engagement Index assesses respondents' satisfaction with their communities and their inclination to volunteer their time, money, and assistance to others. Engaged citizens are positive about the communities they live in and actively give back to them.
Well-Being Indexes: Thriving, Struggling, Suffering
The Thriving, Struggling, and Suffering Indexes measure respondents' perceptions of where they stand, now and in the future. Individuals who rate their current lives a "7" or higher AND their future an "8" or higher are "Thriving." Individuals are "Suffering" if they report their current AND future lives as a "4" and lower. All other individuals are "Struggling."
Well-Being Indexes: Positive Experience
The Positive Experience Index is a measure of experienced well-being on the day before the survey. Questions provide a real-time measure of respondents' positive experiences.
Well-Being Indexes: Negative Experience
The Negative Experience Index is a measure of experienced well-being on the day before the survey. Questions provide a real-time measure of respondents' negative experiences.
National Institutions Index
The National Institutions Index measures confidence in key national institutions prominent in a country's leadership: the military, the judicial system, the national government, and the honesty of elections.
The Corruption Index measures perceptions in a community about the level of corruption in business and government.
Youth Development Index
The Youth Development Index measures a community's focus on the welfare of its children. This index includes general measures of development of youth and respect for youth, along with satisfaction with the educational system.
Community Basics Index
The Community Basics Index measures satisfaction with aspects of everyday life in a community, including education, environment, healthcare, housing, and infrastructure.
The Diversity Index measures a community's acceptance of people from different racial, ethnic, and cultural groups.
The Optimism Index measures a respondent's positive attitude about the future. Specifically, respondents are asked whether certain aspects of their life are getting better or getting worse.
The Communications Index measures the degree to which respondents are connected via electronic communications.
The Violence Index measures a respondent's acceptance of others' use of violence as a means to an end.
The Religiosity Index is a measure of the importance of religion in respondents' lives and their self-reported attendance of religious services. For religions in which attendance at services is limited, care must be used in interpreting the data.