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Count up the number of obseravtions made on the experimental units. That is the sample size.
If the population is of size N, then you allocate the numbers 1 to N to them: one per element of the population. Then generate random numbers in the range 1 - N. The element whose number is thrown up by the generator is in the sample. In the unlikely event that a number is repeated, you ignore the repeat and continue drawing the sample until you have the required correct number in the sample.
Business can randomly sample a group of people in the populations. They can also call for volunteers such as setting up a focus group.
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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.