Retention relative to a standard; r = t_R´/t_R(st)´ = k/k_st, where t_R´ is the adjusted retention time of the component of interest, t_R(st)´ is the adjusted retention time of the standard, k and k_st are the corresponding retention factors. For two adjacent peaks, α expresses the relative retention and is called separation factor (formerly called selectivity or selectivity factor); calculated as 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.