# Local Sensitivity Analysis (Automatic Differentiation)

Sensitivity analysis, or automatic differentiation of the solver, is provided by the DiffEq suite. The model sensitivities are the derivatives of the solution $u(t)$ with respect to the parameters. Specifically, the local sensitivity of the solution to a parameter is defined by how much the solution would change by changes in the parameter, i.e. the sensitivity of the ith independent variable to the jth parameter is $\frac{\partial u_i}{\partial p_{j}}$.

Sensitivity analysis serves two major purposes. On one hand, the sensitivities are diagnostics of the model which are useful for understand how it will change in accordance to changes in the parameters. But another use is simply because in many cases these derivatives are useful. Sensitivity analysis provides a cheap way to calculate the gradient of the solution which can be used in parameter estimation and other optimization tasks.

There are three types of sensitivity analysis. Local forward sensitivity analysis directly gives the gradient of the solution with respect to each parameter along the time series. The computational cost scales like N*M, where N is the number of states and M is the number of parameters. While this gives all of the information, it can be expensive for models with large numbers of parameters. Local adjoint sensitivity analysis solves directly for the gradient of some functional of the solution, such as a cost function or energy functional, in a manner that is cheaper when the number of parameters is large. Global Sensitivity Analysis methods are meant to be used for exploring the sensitivity over a larger domain without calculating derivatives and are covered on a different page.

## Installation and Usage

This functionality does not come standard with DifferentialEquations.jl. To use this functionality, you must install SciMLSensitivity.jl:

]add SciMLSensitivity
using SciMLSensitivity

For complete information on using the sensitivity analyis features, please consult the SciMLSensitivity.jl documentation