What sampling method does the core allocation type use?

Which of the following sampling methods is used for core allocation types?

Simple random sampling: Simple random sampling is to select a part of observation units in the population by completely random method to form a sample (that is, the probability of each observation unit being selected into the sample is the same).

The common method is to number all the observation units in the population first, and then draw some observation units from them to form samples by lottery, random number table or computer-generated random number.

Its advantages are simple and intuitive, and the calculation of mean (or rate) and its standard error is simple and convenient; The disadvantage is that when the group is large, it is difficult to number the individuals in the group one by one, and the samples taken are scattered, so it is difficult to organize the investigation.

Systematic sampling: systematic sampling is also called equidistant sampling or mechanical sampling, that is, all individuals in the group are sorted and numbered according to the characteristics unrelated to the research phenomenon; Then, according to the sample content, the sampling interval k is specified; Randomly select I (i(i