Sampling Frame – A list, map or other specification of the units, which constitute the available information relating to the population designated for a particular sampling scheme. There is a frame corresponding to each state of sampling in a multi-stage sampling scheme. The frame may or may not contain information about the size or other supplementary information of the units, but it should have enough details so that a unit, if included in the sample, may be located and taken up for inquiry. The nature of the frame exerts a considerable influence over the structure of a sample survey. It is rarely perfect, and may be inaccurate, incomplete, inadequately described, out of date or subject to some degree of duplication. Reasonable reliability in the frame is a desirable condition for the reliability of a sample survey based on it. In multi-stage sampling it is sometimes possible to construct the frame at higher stages during the progress of the sample survey itself For example, certain first stage units may be selected in the first instance, and then more detailed lists or maps be constructed by compilation of available information or by direct observation only of the first-stage units actually selected
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