The cephalization factor and the subsequent encephalization quotient was developed by H.J. Jerison in the late 1960s. The formula for the curve varies, but an empirical fitting of the formula to a sample of mammals gives formula_1. As this formula is based on data from mammals, it should be applied to other animals with caution. For some of the other vertebrate classes the power of rather than is sometimes used, and for many groups of invertebrates the formula may give no meaningful results at all.