A thermodynamic factor that is a measure of relative retention of two substances. Formerly called selectivity or selectivity factor. The relative retention; α = t_R2´/t_R1´ = k₂/k₁, where t_R2´ and t_R1´ are the adjusted retention times of peaks 2 and 1, respectively, and k₂ and k₁ are the corresponding retention factors.