Similar to the polar sensitivity factor, the steric sensitivity factor δ for a new reaction series will describe to what magnitude the reaction rate is influenced by steric effects.  When a reaction series is not significantly influenced by polar effects, the Taft equation reduces to:
A plot of the ratio of the rates versus the E value for the substituent will give a straight line with a slope equal to δ.  Similarly to the Hammett ρ value, the magnitude of δ will reflect to what extent a reaction is influenced by steric effects:
*A very steep slope will correspond to high steric sensitivity, while a shallow slope will correspond to little to no sensitivity.
Since E values are large and negative for bulkier substituents, it follows that:
*If δ is positive, increasing steric bulk decreases the reaction rate and steric effects are greater in the transition state.
*If δ is negative, increasing steric bulk increases the reaction rate and steric effects are lessened in the transition state.