The theory of probability sampling, on which sample survey techniques are based, assumes the existence of a list of units from which the sample can be drawn. Such a list is called a frame. A frame is either a complete list of all units of the population or some other basis that provides a selection process such that every element of the population has a known non-zero probability of being included in the sample.
Definition: A sampling frame is a list of units or group of units of the population to be sampled, organized and arranged in such manner that every unit occurs once and only once in the list and no unit is excluded from the list.
In sampling problems, we encounter two types of sampling frames:
(a) Frames for area sampling and
(b) Frames that are lists; for example, lists of households or addresses of housing units
Area frames are usually used to sample geographical areas. With this technique, each element of the population is associated with a particular geographical area consisted by a group of people or households. Then a sample of areas is included in the survey or a sample of these elements is included. A list-sampling frame, on the other hand, is a complete list of well-defined reporting units. This list should contain relevant information about individual units, which will enable efficient sampling.
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