Learn in the textbookPopulation and Sampling
DefinitionChoosing the part of a population to be surveyed (the sample) is called sampling (標本抽出).
ExplanationA survey that examines only the chosen sample is a sample survey; it saves the time and cost of examining everything while still letting you estimate the population from the results. Results shift somewhat depending on which elements happen to be chosen, and this variation is called sampling error. Samples picked by the surveyor's own judgment tend to be biased, so random sampling, which leaves the choice to chance, is the standard approach.
AnalogyIt is like checking a big batch of cookies by taking a few from the pile as they come out of the oven. Pick only the nicest-looking ones and you will misjudge the whole batch.
Memory hookThe Japanese word for a sample, 標本, is the same word used for an insect specimen: an example kept at hand to represent the whole.