3-target population - elements 3.TargetPopulations,Sampling...

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29/1/2013 1 3. Target Populations, Sampling Frames, and Coverage Error 1 The fundamental units of populations are called elements . In household population, most of the time, elements are persons who live in household. In business surveys, the element is an establishment. 2 The target population is the group of elements for which the survey investigator wants to make inferences by using the sample statistics. Target populations f Are finite in size Have some time restrictions Are observable 3 There are often restrictions placed on a survey data collection operation that limit the target population even though elements are well defined at planning stage. 4 In the National Health Survey 2010, Singapore, elements are citizens and permanent residences who are 18 to 79 years old in 2010 and residing in Singapore. But those who were in prisons, long term care medical facilities are excluded. This restricted population is sometimes called survey population. Even though it is not the target population, but it is realistically the actual population from which the survey data are collected. 5 Sampling frame or a set of materials, is used to identify the elements of the target population. Possible sampling frame: Maps of areas List of dwellings List of administrative records List of establishments in Singapore Registrar of Companies The simplest sampling frame consists of a simple list of population elements. 6
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29/1/2013 2 When available sampling frames miss the target population partially or entirely, there are two options to take: 1. Redefine the target population to fit the frame better. 2. Admit the possibility of coverage error in statistics describing the original target population. 7 A sampling frame is perfect when there is a one to one mapping of frame elements to target population elements. There are four possible problems of a sampling frame when it is not perfect: d i li ibl it d li ti undercoverage, ineligible units, duplication and clustering. 8 Undercoverage: There are elements in the target population that do not appear in the sampling frame. Ineligible units: Units in the frame that are not in the target population. Duplication: Several frame units are mapped onto the single element in the target population. Clustering: Multiple elements in the target population are linked to the sample single frame element. 9 Undercoverage Undercoverage produces errors of nonobservation in survey statistics from failure to include parts of the target population in any survey. Example. When fixed line telephones are used to construct the sampling frame for household survey, poorer households without fixed line telephone will be exclude from the frame. With new technology, some households might use mobile phone for communication instead of fixed line telephone. They are excluded as well.
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