# Closed-form solution for an integral involving the p.d.f. and c.d.f. of a $N(0,1)$-distributed random variable

Let $\phi(\cdot)$ and $\Phi(\cdot)$ be the probability and cumulative density functions, respectively, of a random variable with distribution $\text{N}(0,\,1)$. That is,
$$\forall x\in\mathbb{R}:\,\phi(x)=\frac{1}{\sqrt{2\cdot\pi}}\cdot\text{e}^{-x^2/2}$$ and
$$\forall x\in\mathbb{R}:\,\Phi(x)=\int_{-\infty}^{x}\phi(u)\,\text{d}u.$$ I was wondering if you could help me to compute
$$\int_{-\infty}^{w}\phi(x)\cdot\Phi(a+b\cdot x)\,\text{d}x\mbox{,}$$ where $(a,\,b,\,w)\in\mathbb{R}^3$ and $b\neq 0$, please.
Thanks a lot for your help.

• How did this integral come up? Do you expect it to have some kind of closed-form solution? – David Roberts Oct 20 '17 at 6:42
• Hi, @DavidRoberts. This integral is part of an algorithm I am programming in R-software. My purpose is to avoid the use of integrals in R-software because they reduce the efficiency of my algorithm. For this reason, I need to find a closed-form solution for this integral. – Student1981 Oct 20 '17 at 10:09
• Both Maple and Mathematica fail with it. Wanting is not harmful, harmful not want to. – user64494 Oct 20 '17 at 10:51

This integral can be found in D. B. Owen (1980) A table of normal integrals, Communications in Statistics - Simulation and Computation, 9:4, 389-419:

BvN denotes the bivariate normal probability function.

Check in R:

> a <- 2
> b <- 3
> w <- 5
> f <- function(x) dnorm(x)*pnorm(a+b*x)
> integrate(f, lower=-Inf, upper=w)
0.7364551 with absolute error < 1.3e-06
>
> rho <- -b/sqrt(1+b^2)
> Sigma <- cbind(c(1,rho),c(rho,1))
> mvtnorm::pmvnorm(upper=c(a/sqrt(1+b^2), w), sigma=Sigma)
[1] 0.7364551
attr(,"error")
[1] 1e-15
attr(,"msg")
[1] "Normal Completion"


Alternatively, you can express this integral with the Owen $T$-function:

> library(OwenQ)
> 1/2*(pnorm(a/sqrt(1+b^2))  + pnorm(w) - 2*OwenT(w, (b*w+a)/w) - 2*OwenT(-a/sqrt(1+b^2), (a*b+w*(1+b^2))/a) - (a <= 0))
[1] 0.7364551


Benchmark:

> library(mvtnorm)
> library(OwenQ)
> library(microbenchmark)
>
> a <- 2
> b <- 3
> w <- 1
>
> microbenchmark(
+   integral = integrate(function(x) dnorm(x)*pnorm(a+b*x), lower=-Inf, upper=w),
+   mvtnorm = {rho <- -b/sqrt(1+b^2); pmvnorm(upper=c(a/sqrt(1+b^2), w), sigma=cbind(c(1,rho),c(rho,1)))},
+   OwenT = 1/2*(pnorm(a/sqrt(1+b^2))  + pnorm(w) - 2*OwenT(w, (b*w+a)/w) - 2*OwenT(-a/sqrt(1+b^2), (a*b+w*(1+b^2))/a) - (a <= 0))
+ )
Unit: microseconds
expr     min       lq      mean   median       uq      max neval cld
integral  80.677  83.5860 116.97275  90.0240  93.0625 2878.062   100  b
mvtnorm 320.550 327.0625 339.22625 330.3975 336.0315  595.829   100   c
OwenT  22.682  24.6360  28.89006  29.2685  31.9955   51.015   100 a

• @Student1981 I've just added a speed comparison in my answer. OwenT is the way to go. – Stéphane Laurent Oct 21 '17 at 10:18
• These are efficient reductions of the integral to standard libraries. But I'd say a solution with the bivariate normal CDF or OwenT is not in closed form; I would restrict the term "closed-form" to quantities that can be calculated with at most a single integral of elementary functions. – Matt F. Oct 22 '17 at 1:03
• I appreciate your observation, @MattF. I believe I did not choose well my words when I wrote "closed-form solution". I will be more careful next time. However, for my purposes, the approach of Stéphane Laurent's answer is good enough. – Student1981 Oct 22 '17 at 2:21
• FYI, the condition in the OwenT derivation is wrong. It should be -(a/w<0) and not -(a<=0). The conclusion can be derived from 10,010.3 of Owen's paper and using the property $T(u,v) + T(uv,1/v) = \frac{1}{2} (\Phi(u) + \Phi(uv)) - \Phi(u) \Phi(uv) - \frac{1}{2} [v<0]$. – jvdillon Feb 4 at 5:34

There is no known closed-form solution for this integral, though many have asked for it. However, for the case of $w=\infty$ you can use Geller and Ng (http://nvlpubs.nist.gov/nistpubs/jres/73B/jresv73Bn1p1_A1b.pdf), integral 4.3.13.