Similar to ρ values for Hammett plots, the polar sensitivity factor ρ* for Taft plots will describe the susceptibility of a reaction series to polar effects.  When the steric effects of substituents do not significantly influence the reaction rate the Taft equation simplifies to a form of the Hammett equation:
The polar sensitivity factor ρ* can be obtained by plotting the ratio of the measured reaction rates (k) compared to the reference reaction () versus the σ* values for the substituents.  This plot will give a straight line with a slope equal to ρ*.  Similar to the Hammett ρ value:
* If ρ* > 1, the reaction accumulates negative charge in the transition state and is accelerated by electron withdrawing groups.
* If 1 > ρ* > 0, negative charge is built up and the reaction is mildly sensitive to polar effects.
* If ρ* = 0, the reaction is not influenced by polar effects.
* If 0 > ρ* > −1, positive charge is built up and the reaction is mildly sensitive to polar effects.
* If −1 > ρ*, the reaction accumulates positive charge and is accelerated by electron donating groups.