Whenever it is impractical to measure the characteristics of interest of each member of the population. For example, the populations of most countries are too large for any of the characteristics of all of the people within them to be measured. For that reason, sampling techniques are applied so that representative samples can be obtained of country populations.
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A questionnaire has little to do with sampling technique. Sampling technique is to do with who gets the questionnaire and that can be any sampling technique: the questionnaire can be sent to everyone (census), to a random sample, stratified random samples, to random samples in clusters, by quota or convenience. Or a pile of questionnaires can be left for respondents to pick up - self-selection.
Disadvantages of systematic sampling: © The process of selection can interact with a hidden periodic trait within the population. If the sampling technique coincides with the periodicity of the trait, the sampling technique will no longer be random and representativeness of the sample is compromised.
So-called accidental sampling. Please see the link.
Sampling gives good insight of the choosen sample
an approach to sampling that has the characteristics of being randomly selected and the use of probability theory to evaluate sample results. Whereas non-statistical sampling is therefore any sampling approach that does not have both of the characteristicss of statistical sampling. I hope this will help....