In the normal practice of medicine a great deal of information is handled, an example of this is medical records. The first problem that the medical professional is faced with is that he/she has a large quantity of information, without necessarily knowing how best to summarise it. Thus, it is at this point of the epidemiological research that Statistics comes in. Statistics is a discipline which uses mathematical resources to organise and summarise a great deal of data obtained from reality, and infer conclusions with regard to such data. With this series of articles we seek to introduce the clinician to the first steps in the use and handling of numerical data: to distinguish and classify the study variables, to show how to organise and tabulate the measurements obtained by means of the construction of frequency tables, to produce an image that can show results etc, graphically. All these mathematical concepts will be accompanied by practical examples in the field of allergology. BASIC DEFINITIONS