Learn in the textbookPopulation and Sampling
DefinitionSimple random sampling (単純無作為抽出) chooses a sample by chance, for example with random numbers, so that every element of the population has the same probability of being selected.
ExplanationYou prepare a numbered list of every element in the population and draw numbers at random to decide the sample. Because the surveyor's preferences play no part, it prevents bias from the way elements are chosen. On the other hand, it needs a list of the entire population, and when the targets are spread over a wide area, visiting them takes a lot of effort.
AnalogyIt is like putting a slip with everyone's name into a box, shaking it well, and drawing with your eyes closed.
Memory hookThe Japanese 無作為 reads as "without contrivance": the characters say outright that the chooser's intentions are kept out.