Transition state theory requires a value of the transmission coefficient, called  in that theory. This value is often taken to be unity (i.e., the species passing through the transition state  always proceed directly to products  and never revert to reactants  and ). To avoid specifying a value of , the rate constant can be compared to the value of the rate constant at some fixed reference temperature (i.e., ) which eliminates the  factor in the resulting expression if one assumes that the transmission coefficient is independent of temperature.