Multi-Phase Sampling
A multi phase sample should not be confused with a multi-stage sample The former involves a design where some information is collected from the entire sample and additional information is collected from only a part of the original sample Suppose a survey is undertaken to determine the nature and extent of health facilities available in a city and the general opinion of the people. In the first phase a general questionnaire can be sent out to ascertain who amongst the respondents had at one time or other used the hospital services. Then, in the second stage, a comprehensive questionnaire may be sent to only these respondents to ascertain what they feel about the medical facilities in the hospitals. This is a two-phase or double sampling.
The main point of distinction between a multi-stage and a multi-phase sampling is that in the former each successive stage has a different unit of sample whereas in the latter the unit of sample remains unchanged though additional information is obtained from a sub-sample.
The main advantage of a multi-phase sampling is that it effects economy in time, money and effort. In our earlier example, if a detailed questionnaire is sent out to a large sample comprising individuals, they would not be able to provide the necessary information. Second, more time will be required. Finally, it will be far more expensive to carry out the survey, especially when personal interviews are involved.
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