ABSTRACT
The objective of statistics is the use of sample information to infer the nature of a population. For example, laboratory diagnoses about the state of our health are made from a few drops of blood. This procedure is based on the assumption that the circulating blood is always well mixed and that one drop tells the same story as another. In this regard, selecting a sample is not very important, because the material from which we are sampling is uniform and any sample gives similar results. But that is not the case always: sometimes the material is far from uniform. In those instances, it is important to know how to choose a representative sample that exhibits characteristics similar to those possessed by the population about which we wish to make inferences. The scientific way to meet this goal is to select a sample in such a way that every sample of same size has an equal probability of being selected. This statistical procedure based on probability is called the random sampling. In theory, population items are assumed to be well mixed before choosing a representative random sample.
