qsnorm_fast: Fast Approximation of Skew-Normal Quantile Function

In INLAvaan: Approximate Bayesian Latent Variable Analysis

Fast Approximation of Skew-Normal Quantile Function

Description

A fast approximation of skew-normal quantiles using the high-performance approximation algorithm from the INLA GMRFLib C source, and originally by Thomas Luu (see details for reference).

Usage

qsnorm_fast(p, xi = 0, omega = 1, alpha = 0)

Arguments

p	Vector of probabilities.
xi	Location parameter (numeric vector).
omega	Scale parameter (numeric vector).
alpha	Shape parameter (numeric vector).

Details

This function implements a high-performance approximation for the skew-normal quantile function based on the algorithm described by Luu (2016). The method uses a domain decomposition strategy to achieve high accuracy (< 10^{-7} relative error) without iterative numerical inversion.

The domain is split into two regions:

• Tail Regions: For extreme probabilities where \vert u \vert is large, the quantile is approximated using the Lambert W-function, W(z), solving z = \Phi(q) via asymptotic expansion:

  q \approx \sqrt{2 W\left(\frac{1}{2\pi (1-p)^2}\right)}

• Central Region: For the main body of the distribution, the function uses a high-order Taylor expansion of the inverse error function around a carefully selected expansion point $x_0$:

  \Phi^{-1}(p) \approx \sum_{k=0}^5 c_k (z - x_0)^k

This approach is significantly faster than standard numerical inversion (e.g., uniroot) while maintaining sufficient precision for most statistical applications.

Value

Vector of quantiles.

References

Luu, T. (2016). Fast and accurate parallel computation of quantile functions for random number generation (Doctoral thesis). UCL (University College London). https://discovery.ucl.ac.uk/1482128/

Examples

qsnorm_fast(c(0.025, 0.5, 0.975))
qsnorm_fast(c(0.025, 0.5, 0.975), xi = 2, omega = 0.5, alpha = 1)

INLAvaan documentation built on Oct. 2, 2026, 1:07 a.m.