(1) Simple random sampling: including direct lottery, lottery and random number table.
(2) Classified sampling: also called type sampling or stratified sampling, all units in the population are first divided into several categories (groups) according to a certain sign, and then samples are randomly selected from each category.
(3) Mechanical sampling: firstly, the sampling units are queued in a certain order, and the sampling interval (sampling distance) is calculated according to the total number of units and the number of sample units, and then the sample units are selected at a certain interval. Because the sampling interval is equal, it is also called equidistant sampling.
(4) Cluster sampling: firstly, divide the crowd into several groups or groups, and then select them in groups, each group contains several sample units.
Simple random sampling, also known as simple random sampling, pure random sampling and SRS sampling, refers to a sampling method that randomly selects n units from the total number of n units as samples, so that the probability of each possible sample being selected is equal. The characteristics of simple random sampling are: the probability of each sample unit being extracted is equal, and each unit of the sample is completely independent, and there is no certain correlation and exclusion between them.
(1) Simple random sampling requires that the total number of samples n to be sampled is limited.
(2) The number n of simple random samples is less than or equal to the number n of the sample population.
(3) Extract simple random samples from the population one by one.
(4) Simple random sampling is a kind of sampling that cannot be put back.
(5) The probability of each individual being systematically sampled is n/n. ..