Reactivity ratios are a measure of the propensity of a monomer to react with itself compared to reaction with a second monomer. Two ratios, rl and r2, are assigned when two monomers are involved, each assigned for one of the monomers. A reactivity ratio of unity indicates that a given monomer radical is equally as likely to react with itself as with the comonomer; values larger than unity indicate an increasing tendency for a monomer radical to react with itself, while values less than unity indicate an increasing tendency to react with the comonomer. As the polymer chain grows, the reactivity ratio for the monomer "last on" the chain, and present in its radical form, determines the likelihood for reaction with another of its type. Alfrey and Price  further related the reactivity ratios to a set of constants, Q and e. Constant Q is related to the reactivity of the radical adduct formed from a given monomer as (primarily) influenced by resonance stabilization, while constant e is related to the polar effects of substituent groups in the monomer .