The enhanced resonance parameter, , is a measure of the influence of resonance on a new reaction. When ,  the resonance effects for a particular reaction are no different from those for reaction of the unsubstituted reference compound. However, when , the reaction in question is more sensitive to resonance effects than the standard, and when , the reaction is less sensitive to such effects.
The enhanced resonance parameter is determined by first establishing the Hammett Reaction constant from data collected from meta-substituted compounds, and subsequently correlating the remaining data to fit the modified equation described above.