What are the common sampling methods?

Simple random sampling: also known as simple random sampling, pure random sampling and SPS 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. Systematic sampling: also known as mechanical sampling or systematic sampling. It is to arrange the whole units into a graph or list according to a certain sign or order, and then extract the sample units at equal distance or interval. Stratified sampling: firstly, the unit of the whole population is divided into several sub-populations (layers) according to certain characteristics, and then simple random sampling is carried out from each layer to form a statistical calculation method of samples.