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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.
Generally speaking, assuming that a population contains n individuals, if a sample is extracted from it one by one, and the probability of each individual being extracted is equal, then such a sampling method is called simple random sampling.
trait
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.
Simple random sampling requires that the total number of samples sampled, n, is limited. The numb n of simple random samples is less than or equal to that numb n of the sample population. Extract simple random samples from the population one by one. The probability of each individual being systematically sampled is n/n.
limit
1, it is time-consuming and laborious to number the research objects in advance.
2. The overall distribution is scattered, which will make the distribution of sampling samples scattered and bring difficulties to research.
3. When the sample size is small, there may be deviation, which will affect the representativeness of the sample.
4. When it is known that a certain characteristic of the research object will directly affect the research results, if we want to control it, we can't use simple random sampling method.
sampling method
The most basic sampling method of simple random sampling. Divided into repeated sampling and non-repeated sampling. In repeated sampling, the units extracted each time still return to the population, and the units in the sample may be extracted several times. In non-repeated sampling, the extracted units will not be put back into the population, and the units in the sample can only be extracted once. Non-repeated sampling was used in social survey.
The specific methods of simple random sampling are:
Direct selection method
Direct sampling method, that is, direct random sampling from the population. For example, a number of goods are randomly selected from the shelves for inspection; Choose several stalls from the farmer's market stalls for investigation or visit.
draw lots
Firstly, all individuals in the population are numbered (the numbering range can be 1 to n), and the numbers are written on numbers with the same shape and size, which can be balls, cards, paper strips, etc. Then put these numbers in the same box and mix them evenly.
When drawing lots, every time 1 number is drawn, a sample with a capacity of is obtained. When numbering individuals, you can also use the existing number, for example, when taking samples from the whole class, you can use the student's student number and seat number. The lottery method is simple and easy, and it is suitable for the situation that there are not many individuals in the whole population.