Statistical parameters, usually used for diagnostic procedures, in many cases cannot be considered to be consistent ones from the statistical point of view, being strongly dependent on sample size. It leads to considerable devaluation of diagnostic results. This paper concerns the problem of consistency verification of parameters in the initial (pre-classification) stage of research. A complete list of parameters, which may be useful for description of text lexicostatistical structure, was determined. Each of these parameters was exposed to the justifiability test. In the result, a number of consistent parameters have been selected, which represent a description tool for the system characteristics of any text and corpora. Having rapid speed of convergence to the limit values, they may effectively perform classification procedures on text data of the arbitrary size. The proposed model of approximation makes it possible as well to forecast the values of all parameters for any sample size.