Worldwide Research Methodology

To ensure that the Gallup worldwide survey data are representative of 98% of the world's adult population, the following methodology is employed in every country:

Coverage

The target population is the entire civilian, non-institutionalized population, age 15 and older. The coverage area is the entire country, including rural areas, and the sampling frame represents the entire non-institutional civilian population.

The typical survey includes at least 1,000 individuals. In some countries, Gallup collects oversamples 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.

Questions

Gallup uses a standard set of core questions worldwide. In some regions, Gallup asks supplemental region-specific questions. For example, Gallup modifies the questions it uses in heavily indebted poor countries toward providing information about progress on the Millennium Development Goals. Gallup asks all respondents demographic questions such as age, gender, education, and income.

Translations

The questionnaire is translated into the major languages of each country. 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.

Interviewer Training

Gallup trains interviewing supervisors and interviewers on administering the questionnaire and the execution of field procedures. This interviewing training usually takes place in a central location.

Survey Mode

Gallup uses telephone surveys 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, Gallup uses a random-digit-dial (RDD) method or a nationally representative list of phone numbers. 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, Gallup uses an area frame design for face-to-face interviewing in randomly selected households.

Face-to-face interviews are approximately 1 hour, while telephone interviews are about 30 minutes.

Sampling

With some exceptions, all samples are probability based and nationally representative of the resident population aged 15 and older. The coverage area is the entire country including rural areas, and the sampling frame represents the entire civilian, non-institutionalized, aged 15 and older population of the country. Exceptions include areas where the safety of the interviewing staff is threatened and scarcely populated islands in some countries. The country details document outlines methodology descriptions, including sample exclusions, for each country. Sampling procedures include the following stages:

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, 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, Gallup uses simple random sampling.

In countries where telephone interviewing is employed, Gallup uses a RDD method or a nationally representative list of phone numbers. In select countries where cell phone penetration is high, Gallup uses a dual sampling frame. Gallup makes at least three attempts to reach a person in each household.

STEP 2 -- Selecting Households: Gallup uses random route procedures 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, interviewers make attempts at different times of the day, and when possible, on different days. If the interviewer cannot obtain an interview at the initial sampled household, he or she uses a simple substitution method.

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. In a few Middle East and Asian countries, gender-matched interviewing is required, and probability sampling with quotas is implemented during the final stage of selection. Gallup implements quality control procedures to validate the selection of correct samples and that the correct person is randomly selected in each household.

Statistical Validity

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 at 50% is ±3 percentage points. The country details document lists a margin of error for each country survey. The margin of error reflects the influence of weighting.

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.

In addition to sampling error, question wording and practical difficulties in conducting surveys can affect survey validity.

For more detailed information or any questions regarding Gallup's worldwide research methodology, contact Gallup.