Briefly answer the specific classification of simple random sampling and their respective characteristics.

Answer: Simple random sampling is the most basic random sampling method. Before sampling, the whole unit is not grouped or arranged, and samples are taken from the whole according to the principle of randomness. The specific pumping methods are as follows:

(1) direct sampling method: it is to directly and randomly select samples from the survey population for investigation. This method is suitable for sampling the population concentrated in a small space.

(2) Draw lots: write the numbers of all units on the paper with the same shape, and then draw them randomly. The units represented by the extracted numbers are random samples until a predetermined number of samples are extracted.

(3) Random number table method: numbering the whole unit, determining the numbers of several columns or rows in the random number table according to the maximum number of digits, that is, the total number of units, then counting in any direction from the first digit of any row or column, and designating the digits falling within the total number of units as sample units until a predetermined number of sample units are extracted.