Swain–Lupton equation replaces Hammett equation
Redefining the equation
C. Gardner Swain and Elmer C. Lupton Jr. from the Massachusetts Institute of Technology redefined the substituent parameter, σ, based on the idea that no more than two variables (resonance effects and field effects) are necessary to describe the effects of any given substituent.  Field effects, F,  are defined to include all effects (inductive and pure field). Likewise, effects due to resonance, R,  are due to the average of electron-donating ability and electron-accepting ability. These two effects are assumed to be independent of each other and therefore can be written as a linear combination: