# Advantages of Box-Muller over inverse CDF method for simulating Normal distribution?

In order to simulate a normal distribution from a set of uniform variables, there are several techniques:

1. The Box-Muller algorithm, in which one samples two independent uniform variates on $(0,1)$ and transforms them into two independent standard normal distributions via:

2. the CDF method, where one can equate the normal cdf $(F(Z))$ to a Uniform variate:

and derive

My question is: which is computationally more efficient? I would think
its the latter method – but most of the papers I read use Box-Muller –
why?

The inverse of the normal CDF is know and given by:

Hence:

From a purely probabilistic perspective, both approaches are correct and hence equivalent. From an algorithmic perspective, the comparison must consider both the precision and the computing cost.

Box-Muller relies on a uniform generator and costs about the same as this uniform generator. As mentioned in my comment, you can get away without sine or cosine calls, if not without the logarithm:

• generate $$U1,U2iid∼U(−1,1)U_1,U_2\stackrel{\text{iid}}{\sim}\mathcal{U}(-1,1)$$ until $$S=U21+U22≤1S=U_1^2+U_2^2\le 1$$
• take $$Z=√−2log(S)/SZ=\sqrt{-2\log(S)/S}$$ and define $$X1=ZU1, X2=ZU2X_1=ZU_1\,,\ X_2=Z U_2$$

The generic inversion algorithm requires the call to the inverse normal cdf, for instance qnorm(runif(N)) in R, which may be more costly than the above and more importantly may fail in the tails in terms of precision, unless the quantile function is well-coded.

To follow on comments made by whuber, the comparison of rnorm(N)and qnorm(runif(N))is at the advantage of the inverse cdf, both in execution time:

> system.time(qnorm(runif(10^8)))
sutilisateur     système      écoulé
10.137           0.120      10.251
> system.time(rnorm(10^8))
utilisateur     système      écoulé
13.417           0.060      13.472


using the more accurate R benchmark

        test replications elapsed relative user.self sys.self
3 box-muller          100   0.103    1.839     0.103        0
2    inverse          100   0.056    1.000     0.056        0
1    R rnorm          100   0.064    1.143     0.064        0


and in terms of fit in the tail:

Following a comment of Radford Neal on my blog, I want to point out that the default rnorm in R makes use of the inversion method, hence that the above comparison is reflecting on the interface and not on the simulation method itself! To quote the R documentation on RNG:

‘normal.kind’ can be ‘"Kinderman-Ramage"’, ‘"Buggy
Kinderman-Ramage"’ (not for ‘set.seed’), ‘"Ahrens-Dieter"’,
‘"Box-Muller"’, ‘"Inversion"’ (the default), or ‘"user-supplied"’.
(For inversion, see the reference in ‘qnorm’.)  The
Kinderman-Ramage generator used in versions prior to 1.7.1 (now
called ‘"Buggy"’) had several approximation errors and should only
be used for reproduction of old results.  The ‘"Box-Muller"’
generator is stateful as pairs of normals are generated and
returned sequentially.  The state is reset whenever it is selected
(even if it is the current normal generator) and when ‘kind’ is
changed.