Instantaneous relative rate of change of a function of the individual weight, w, that is, H(w∞)-H(w), where w∞ is the asymptotic individual weight and H(w) is a function of w (frequently a power function, including the logarithmic function). The adopted models for the function H(w) have two constants, w∞ and K. Some models introduce one more parameter, b, which is used to obtain a general relation to include the most common individual growth relations. The constant K has the physical dimension of time.