A Brief Introduction to Sampling Researchers usually cannot make direct observations of every individual in the population they are studying. Ideally, the sample corresponds to the larger population on the characteristic s of interest. In that case, the researcher's conclusions from the sample are probably applicable to the entire population.
Survey Research Sampling Most of our national surveys of the general public are conducted either online with our American Trends Panel or by telephone using a random digit sample of landline and cellphone numbers in the United States. Pew Research Center also conducts international surveys that involve sampling and interviewing people in multiple countries.
Lastly, we sometimes survey special populations, such as foreign policy experts, scientists or journalists. In all of our surveys, we use probability sampling to help ensure adequate representation of the groups we survey. The design of the landline sample ensures representation of both listed and unlisted numbers including those not yet listed by using random digit dialing.
This method uses random generation of the last two digits of telephone numbers selected on the basis of the area code, telephone exchange and bank number.
A bank is defined as contiguous telephone numbers, for example to The telephone exchanges are selected to be proportionally stratified by county and by telephone exchange within the county.
Only banks of telephone numbers containing three or more listed residential numbers are selected. The cellphone sample is drawn through systematic sampling from dedicated wireless banks of contiguous numbers and shared service banks with no directory-listed landline numbers Research sampling ensure that the cellphone sample does not include banks that are also included in the landline sample.
The sample is designed to be representative both geographically and by large and small wireless carriers. Both the landline and cell samples are released for interviewing in replicates, which are small random samples of the larger sample. Using replicates to control the release of telephone numbers ensures that the complete call procedures are followed for the entire sample.
The use of replicates also ensures that the regional distribution of numbers called is appropriate. This also works to increase the representativeness of the sample.
This method of selecting respondents within each household improves participation among young people, who are often more difficult to interview than older people because of their lifestyles. Unlike a landline phone, a cellphone is assumed in Pew Research polls to be a personal device.
This means that, for those in the cell sample, no effort is made to give other household members a chance to be interviewed. Although some people share cellphones, it is still uncertain whether the benefits of sampling among the users of a shared cellphone outweigh the disadvantages.
Elites and other special populations Representative surveys can be conducted with almost any population imaginable. It is common for surveyors to want to collect information from experts or elites in particular fields such as policymakers, elected officials, scientists or news editors and other special populations such as special interest groups, people working in particular sectors, etc.
The principles of drawing a representative sample are the same whether the sample is of the general population or some other group. Decisions must be made about the size of the sample and the level of precision desired so that the survey can provide accurate estimates for the population of interest and any subgroups within the population that will be analyzed.
Some special challenges arise when sampling these populations. In particular, it may be difficult to find a sampling frame or list for the population of interest and this may influence how the population is defined.
In addition, information may be available for only some methods of contacting potential respondents e. If most members in the population of interest have internet access and email addresses are available for contacting them, the web often provides a convenient and inexpensive way to survey experts or other special populations.
Pew Research Center occasionally conducts surveys of opinion leaders, especially those in public policy roles. The opinion of elites is often compared with that of the general public to better determine whether these groups have similar or different opinions.In social science research, snowball sampling is a similar technique, where existing study subjects are used to recruit more subjects into the sample.
Some variants of snowball sampling, such as respondent driven sampling, allow calculation of selection probabilities and are probability sampling methods under certain conditions. Sampling Bias. Author(s) David M. Lane.
Prerequisites. Inferential Statistics (including sampling) Learning Objectives. Recognize sampling bias; Distinguish among self-selection bias, undercoverage bias, and survivorship bias. Sampling. Brooke is a psychologist who is interested in studying how much stress college students face during finals.
The reason process and care in sampling has importance, and deserves real care is: representation and bias. In statistics, quality assurance, and survey methodology, sampling is the selection of a subset (a statistical sample) of individuals from within a statistical population to estimate characteristics of the whole population.
Two advantages of sampling are that the cost is lower and data collection is faster than measuring the entire population. Each observation measures one or more properties.