Rules, formulas and statistical tests have been widely used in studies that use continuous variables in the normal or Gaussian distribution or other tests whose data fit this type of distribution (t and z distributions) and whose parameters can be defined. However, there are cases in which the model and goal of the scientific work allow only the use of discrete or nominal variables. The absence or presence of a structure or organ (muscles, nerves, blood vessels, bones, and ligaments), characteristics associated with these organs, such as the source of nerves, vessels and muscles and the distribution of nerves and vessels are discrete and / or nominal variables that do not fit the Gaussian distribution, nor of z or t, unless the measures of these structures are taken. A simple comparative method within the non-parametric distribution has been proposed, (1) between two different samples from the same species and (2) for two different species, based on the concepts of normal and variation in anatomy and on modal class in statistics here called Comparative Anatomy Index (IAC). The main findings on the use of this method are: (1) to allow the relationship between data from discrete or nominal variables in comparative anatomy or in other areas of morphology, (2) to generate an objective method (numerical) of comparison between samples of structures from the same species and between different species and (3) to fill the lack of a specific method for the statistical work in comparative anatomy or in morphology, in general.