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Undercoverage occurs when the sampling frame does not include all members of the target population. In the previous example, voters are undercovered because not all voters are Twitter users. On the other hand, overcoverage results when some members of the target population are overrepresented in the sampling frame.
Other sampling frames can include employment records, school class lists, patient files in a hospital, organizations listed in a thematic database, and so on. [1] [5] On a more practical levels, sampling frames have the form of computer files. [1] Not all frames explicitly list population elements; some list only 'clusters'.
A visual representation of the sampling process. In statistics, quality assurance, and survey methodology, sampling is the selection of a subset or a statistical sample (termed sample for short) of individuals from within a statistical population to estimate characteristics of the whole population.
A probability-based survey sample is created by constructing a list of the target population, called the sampling frame, a randomized process for selecting units from the sample frame, called a selection procedure, and a method of contacting selected units to enable them to complete the survey, called a data collection method or mode. [10]
An area sampling frame is an alternative to the most traditional type of sampling frames. A sampling frame is often defined as a list of elements of the population we want to explore through a sample survey. A slightly more general concept considers that a sampling frame is a tool that allows the identification and access to the elements of the ...
Such procedures are used to mitigate issues in the sampling ranging from sampling error, under-coverage of the sampling frame to non-response. [ 16 ] : 45 [ 17 ] For example, these methods can be used to make the sample more similar to some target "controls" (i.e., population of interest), a process also called "standardization".
From the sampling frame, a starting point is chosen at random, and choices thereafter are at regular intervals. For example, suppose you want to sample 8 houses from a street of 120 houses. 120/8=15, so every 15th house is chosen after a random starting point between 1 and 15.
Probability samples are highly affected by problems of non-coverage (not all members of the general population have Internet access) and frame problems (online survey invitations are most conveniently distributed using e-mail, but there are no e-mail directories of the general population that might be used as a sampling frame).