# Chapter 5 Monte Carlo integration

A typical usage of simulation of random variables is Monte Carlo integration. With $$X_1, \ldots, X_n$$ i.i.d. with density $$f$$ $\hat{\mu}_{\textrm{MC}} := \frac{1}{n} \sum_{i=1}^n h(X_i) \rightarrow \mu := E(h(X_1)) = \int h(x) f(x) \ \mathrm{d}x$ for $$n \to \infty$$ by the law of large numbers (LLN).

Monte Carlo integration is a clever idea, where we use the computer to simulate i.i.d. random variables and compute an average as an approximation of an integral. The idea may be applied in a statistical context, but it way also have applications outside of statistics and be a direct competitor to numerical integration. By increasing $$n$$ the LLN tells us that the average will eventually become a good approximation of the integral. However, the LLN does not quantify how large $$n$$ should be, and a fundamental question of Monte Carlo integration is therefore to quantify the precision of the average.

This chapter first deals with the quantification of the precision — mostly via the asymptotic variance in the central limit theorem. This will on the one hand provide us with a quantification of precision for any specific Monte Carlo approximation, and it will on the other hand provide us with a way to compare different Monte Carlo integration techniques. The direct use of the average above requires that we can simulate from the distribution with density $$f$$, but that might have low precision or it might just be plain difficult. In the second half of the chapter we will treat importance sampling, which is a technique for simulating from a different distribution and use a weighted average to obtain the approximation of the integral.