The basic principle of sampling is randomness, that is, when sampling, the probability (that is, possibility) of whether each individual in the group is selected is completely equal. Because random sampling makes every individual have an equal chance to be extracted, it is very likely that the sample will keep the same structure as the population, or it is most likely to show some characteristics of the population in the sample, thus ensuring that the population can be inferred from the sample. In addition, following the principle of randomness can budget or control the range of sampling error, so that researchers can objectively evaluate the accuracy of research results and determine the sample size according to the required accuracy.