### 20.4.21 Inverse distribution functions

The icdf command finds the inverse cumulative distribution function for a probability distribution.

- cdf takes one or more mandatory arguments and one optional argument.
  - distd, the name of the function that finds the distribution’s density function.
  - params, any parameters that would normally be passed to distd.
  - x, a number.
- icdf(distd,params) returns an expression for the inverse cumulative distribution function for distd with parameters params.
- icdf(distd,params,x) returns the value of the inverse cumulative distribution function at x.

#### Example

icdf(normald,0,0.5,0.975)

0.97998199227 |  |  |  |  |  |  |  |  |  |