# Generating correlated random deviates

Discussion in 'Math' started by someonesdad, Jul 19, 2010.

1. ### someonesdad Thread Starter Senior Member

Jul 7, 2009
1,585
141
Here's a problem that I'd like to solve to help me with some Monte Carlo modeling. Suppose I have n random variables Xi (indexed by i, i = 1, ..., n). The distributions for each of these random variables is arbitrary; however, practically, I'll be using numpy and python, so I'll use the discrete and continuous distributions that numpy supports.

Now, suppose I also have an n x n correlation matrix R for these random variables; this is a matrix with all 1's on the diagonal, is symmetric, and each off-diagonal element is less than or equal to 1 in absolute value. I want to generate m random vectors with components Xi such that the random samples in each dimension have the requisite covariances implied by the correlation matrix.

Do any of you folks know how to do this? A reference to the literature on how to do it is fine too. An ideal answer would work within the constraints of python/numpy.

2. ### Tesla23 Active Member

May 10, 2009
323
67
Diagonalise the correlation matrix, transform to the basis of eigenvectors, in this representation the samples are uncorrelated so generate your samples here then transform back.

For more info look up the Karhunen-Loev transform.

3. ### someonesdad Thread Starter Senior Member

Jul 7, 2009
1,585
141
Cool! I'd never heard of the transform, but the method (kind of) intuitively rings true. I'll give it a try.

4. ### someonesdad Thread Starter Senior Member

Jul 7, 2009
1,585
141
Tesla23:

I haven't played with this yet, but I'll get to it. My only concern is as follows. Suppose I generate a column vector X where the elements are samples from a chosen group distributions (each row will correspond to an independent variable in the Monte Carlo simulation). Suppose P is the matrix of eigenvectors of the correlation matrix. I transform X to get $Y = P^{-1}XP$. Now I have a set of n transformed vectors Yi; I take these Yi and put them in as columns of a matrix A. The rows of this matrix now form the random samples of the chosen distributions. These rows should have the desired covariance matrix.

Does this diagonalizing similarity transformation change the stochastic properties of the rows of matrix A? In other words, are these rows still random samples from the originally-chosen distributions?

I'll find this out experimentally when I try the technique out, but since you obviously know this stuff, you could short-circuit my work.