N.B.
The sampling unit is not necessarily the same as the study unit.
Sample design:
The scheme for selecting the sampling units from the study population.
Sampling frame:
The list of units from which the sample is to be selected.
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The existence of an adequate and up-to-date sampling frame often
defines the study population.
Sampling methods
An important issue influencing the choice of the most appropriate sampling method is
whether a sampling frame is available, that is, a listing of all the units that compose the study
population.
a) Non-probability sampling methods
Examples:
1. Convenience sampling: is a method in which for convenience sake the study
units that happen to be available at the time of data collection are selected.
2. Quota sampling: is a method that insures that a certain number of sample units
from different categories with specific characteristics appear in the sample so
that all these characteristics are represented. In this method the investigator
interviews as many people in each category of study unit as he can find until he
has filled his quota.
3. Purposeful sampling strategies for qualitative studies: Qualitative research
methods are typically used when focusing on a limited number of informants,
whom we select strategically so that their in-depth information will give optimal
insight into an issue about which little is known. This is called purposeful
sampling.
The above sampling methods do not claim to be representative of the entire
population.
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