This paper deals with a mathematical model which explains process of aquisition or skillfulness of human. Especially, in the case of determinating a ship's position, a degree of aquisitiion or skillfulness has a significant meaning in the view point of reliability of a data because it is closely connected with the position error. In this paper, the authors have obtained the following mathematical model, who analyzed quantitavely the data obtained by a process of measuring the horizontal sextant angles and extracted the factors or parameters affecting the process of aquisition or skillfulness of human. Namely, H=Me-iN M : the coefficiency which represent the degree of difficulty in measuring the object i : personal parameter N : the number of measurements