If the stratified samples are randomly sampled first, then it can be guaranteed that the probability of each individual being sampled after elimination is equal, otherwise it is unequal. Questionnaire sampling methods include probability sampling and non-probability sampling. Stratified sampling method is also called type sampling method. It is a method of randomly selecting samples (individuals) of different layers from the whole that can be divided into different subgroups (or layers) according to the specified proportion. The advantage of this method is that the sample is representative and the sampling error is small. The disadvantage is that the sampling procedure is simpler and more complicated than random sampling. Stratified sampling in quantitative survey is an excellent probability sampling method, which is often used in survey. Probabilistic sampling includes: random sampling (lottery method, random number method, substituted or non-substituted sampling) and stratified sampling (dividing the whole unit into different layers according to certain characteristics or laws, and then randomly selecting a certain number of sampling units from each layer to form samples. ), cluster sampling, systematic sampling. Non-probabilistic sampling: convenient sampling and judgment sampling.