When we study a group, due to the limitation of funds, time, manpower and material resources, it is usually impossible to include all the individuals in the group in the research scope. For example, we want to investigate the sleep time of high school students. You can't get the average sleep time of all high school students, so you have to settle for the second best and collect the sleep time of some high school students. Therefore, in the actual investigation process, some representative individuals are often selected from the research objects and their characteristics are used to reflect the characteristics of the whole object. These selected individuals are the research samples.