parameters, respectively.The model in Equation 4 may also be expressed in the exponential form, MT = aeIHog(A/W), (5)with ~ = b if the log in Equation 5 is to the natural base e and with ~ = b/log2 e if the log in Equation 5 is to base 2. This equivalence between Equations 4 and 5 is immediately apparent by taking logs on each side of the two equations. The model in Equation 3 may also be expressed in a slightly more complex exponential form. In terms of Fitts' index of difficulty, Id, definedin Equation 1, the model in Equation 4 may be expressed as