A sample is a generalization about a population and is represented by a group of units selected from the population (also called a subset of the population). The sample is meant to be representative of the population; thus, in many studies, there are many possible samples. There are also types of samples, such as a matched sample, in which two of the members are paired; an example would be the IQ of twins. There is often a good reason for taking samples of a population: Most of the time, a population is too large to study as a whole.

For example, take the “smaller” example of the above population of 10 cats. Again, none of them are identical, but certain common features between the cats can be measured, including color, fur length, and weight. If data is collected about the fur length of the 10 cats (the population), then if we chose only to take the cats with long fur, that would be a sampling. Another example is the population for a study of physical condition of all children born in the United States in the 1970s; the sample could be all children born on July 5 in any of those years.

A matched sample is a type of sampling method used in statistics. For example, in studying population IQs, identical twins could be paired up to measure and compare intelligence. Photographer’s Choice/Getty Images.