What is the elasticity of substitution formula and how is it used to measure the responsiveness of the substitution between two factors of production?

The elasticity of substitution formula measures how easily one factor of production can be replaced by another in the production process. It is calculated as the percentage change in the ratio of two factors divided by the percentage change in their marginal rate of technical substitution. A higher elasticity indicates that factors are more easily substituted, while a lower elasticity suggests they are less interchangeable.