High Quality Content by WIKIPEDIA articles! A sample is a subject chosen from a population for investigation. A random sample is one chosen by a method involving an unpredictable component. Random sampling can also refer to taking a number of independent observations from the same probability distribution, without involving any real population. A probability sample is one in which each item has a known probability of being in the sample. The sample usually will not be completely representative of the population from which it was drawn? this random variation in the results is known as sampling error. In the case of random samples, mathematical theory is available to assess the sampling error.