# qgeneric: Generic function to find quantiles of a distribution In flexsurv: Flexible Parametric Survival and Multi-State Models

 qgeneric R Documentation

## Generic function to find quantiles of a distribution

### Description

Generic function to find the quantiles of a distribution, given the equivalent probability distribution function.

### Usage

```qgeneric(pdist, p, matargs = NULL, scalarargs = NULL, ...)
```

### Arguments

 `pdist` Probability distribution function, for example, `pnorm` for the normal distribution, which must be defined in the current workspace. This should accept and return vectorised parameters and values. It should also return the correct values for the entire real line, for example a positive distribution should have `pdist(x)==0` for x<0. `p` Vector of probabilities to find the quantiles for. `matargs` Character vector giving the elements of `...` which represent vector parameters of the distribution. Empty by default. When vectorised, these will become matrices. This is used for the arguments `gamma` and `knots` in `qsurvspline`. `scalarargs` Character vector naming scalar arguments of the distribution function that cannot be vectorised. This is used for the arguments `scale` and `timescale` in `qsurvspline`. `...` The remaining arguments define parameters of the distribution `pdist`. These MUST be named explicitly. This may also contain the standard arguments `log.p` (logical; default `FALSE`, if `TRUE`, probabilities p are given as log(p)), and `lower.tail` (logical; if `TRUE` (default), probabilities are P[X <= x] otherwise, P[X > x].). If the distribution is bounded above or below, then this should contain arguments `lbound` and `ubound` respectively, and these will be returned if `p` is 0 or 1 respectively. Defaults to `-Inf` and `Inf` respectively.

### Details

This function is used by default for custom distributions for which a quantile function is not provided.

It works by finding the root of the equation h(q) = pdist(q) - p = 0. Starting from the interval (-1, 1), the interval width is expanded by 50% until h() is of opposite sign at either end. The root is then found using `uniroot`.

This assumes a suitably smooth, continuous distribution.

### Value

Vector of quantiles of the distribution at `p`.

### Author(s)

Christopher Jackson <chris.jackson@mrc-bsu.cam.ac.uk>

### Examples

```
qnorm(c(0.025, 0.975), 0, 1)
qgeneric(pnorm, c(0.025, 0.975), mean=0, sd=1) # must name the arguments
```

flexsurv documentation built on June 18, 2022, 1:06 a.m.