Definición Random sample

The random sample is one of the selection methods for representative samples. In a random sample, the probability of ending up in the sample is the same for each element of a population. An example for a simple random sample is the drawing of a ball from an urn, such as the weekly lottery draw.

In a stratified random sampling, the elements of a population are divided into groups based on a previously determined attribute. Subsequently, a simple random sample is performed within each of those groups. An example would be the random selection of 100 residents from each of the 50 States. 

Other methods of random samplings are cluster samples and multi-stage samples.