A sample is a (small) subset of a population (set).
Statistics are calculated from the sample and used either to describe the population or make inferences about the population.
The process for selecting a sample should ensure that a "random" selection from the population is made. A random selection will occur if each member of the population has an equally likely chance of being chosen for the sample. A sample that is not random is called biased.
When the sample is random, statistics calculated using the sample better describe the population and more accurately make predictions about the population.
There are many types of random samples, some of which are listed below:
The most common type of random sample is the simple random sample.