A sample is a set of data taken using certain procedures from the general population for research analysis. Representativeness is a property of reproducing the concept of the whole in its part. Otherwise, it is the possibility of extending the concept of a part to the whole, which includes this part.
The representativeness of the sample is an indicator consisting in the fact that the sample should fully and reliably display the characteristics of the aggregate of which it is a part. It can also be defined as a property of the sample to most fully represent the characteristics of the population that are significant in terms of the purpose of the study.
Suppose that the general population is all school students (900 people from 30 classes, 30 people in each class). Object of study - the attitude of students to smoking. A sample set consisting of 90 students of only senior classes will present the whole population much worse than a sample of the same 90 students, which would include 3 students from each class. The main reason is the unequal distribution by age. Thus, in the first case, the representativeness of the sample will be low. In the second case - high.
In sociology they say that there is a representativeness of the sample and its non-representativeness.
An example of a non-representative sample is the classic case that occurred in the United States in 1936 during the presidential election.
The Literari Digest magazine, which had previously very successfully predicted the results of previous elections, this time made a mistake in its forecasts, although it sent several million written questions to subscribers and also to respondents whom they selected from phone books and car registration lists. In the 1/4 of the ballots that were returned, the votes were distributed as follows: 57% gave priority to the Republican candidate named Alf Landon, and 41% preferred the current president, Democrat Franklin Roosevelt.
In fact, F. Roosevelt won the election, gaining nearly 60% of the vote. The Literari Digest error was as follows. They wanted to increase the representativeness of the sample . And since they knew that most of their subscribers consider themselves Republicans, they decided to expand the sample at the expense of respondents selected by them from phone books and car registration lists. But they did not take into account the existing realities and actually selected even more supporters of the Republicans, because during the Great Depression, the middle and upper classes could afford to have cars and phones. And these were mostly Republicans, not Democrats.
There are various types of sampling: simple random, serial, typical, mechanical and combined.
A simple random sample consists in selecting from the entire set of studied units at random without any system.
Mechanical sampling is used when there is orderliness in the general population, for example, there is a certain sequence of units (registration numbers of employees, election lists, telephone numbers of respondents, apartment and house numbers, etc.).
Typical selection is used when the entire population can be divided into groups by type. When working with the population, such can be, for example, educational, age, social groups, when researching enterprises - an industry or a separate organization, etc.
Series selection is convenient when units are combined into small series or groups. Such a series can be finished goods parties, school classes, labor collectives and other groups.
Combined sampling involves the use of all previous types of sampling in one or another combination.