Instantaneous relative rate of change of a function of the individual weight, w, that is, H(wOO)-H(w), where wOO 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, wOO 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 -1.