A sample or sample set is a part of the general set of elements that is covered by an experiment (observation, survey). Sample characteristics:
- Qualitative characteristics of the sample – what exactly we choose and what methods of sampling we use for this.
- The quantitative characteristic of the sample is how many cases we select, in other words, the sample size.
Sample size is the number of cases included in the sample. Samples can be conditionally divided into large and small ones, since different approaches are used in mathematical statistics depending on the sample size. It is believed that samples larger than 30 can be classified as large.
The sample may be considered representative or non-representative. The sample will be representative when examining a large group of people, if within this group there are representatives of different subgroups, only in this way can correct conclusions be drawn.
An example of a non-representative sample
In the United States, one of the most famous historical examples of non-representative sampling is the case that occurred during the presidential election in 1936 . The Litrery Digest, which had successfully predicted the events of several previous elections, miscalculated by sending out ten million test ballots to its subscribers, as well as to people selected from countrywide phone books and people from car registration lists. In 25% of the returned ballots (nearly 2.5 million), the votes were distributed as follows:
- 57% preferred Republican candidate Alf Landon
- 40% chose then-Democratic President Franklin Roosevelt
As is well known, Roosevelt won the actual elections with more than 60% of the votes. The Litreary Digest’s mistake was this: wanting to increase the representativeness of the sample—because they knew that most of their subscribers considered themselves Republicans—they expanded the sample with people chosen from phone books and registration lists. However, they did not take into account contemporary realities and in fact recruited even more Republicans: during the Great Depression, it was mostly the middle and upper class (that is, most Republicans, not Democrats) who could afford to own phones and cars.