Generating random variables with given variance-covariance matrix can be useful for many purposes. For example it is useful for generating random intercepts and slopes with given correlations when simulating a multilevel, or mixed-effects, model (e.g. see here). This can be achieved efficiently with the Choleski factorization. In linear algebra the factorization or decomposition of a matrix is the factorization of a matrix into a product of matrices. More specifically, the Choleski factorization is a decomposition of a positive-defined, symmetric1 matrix into a product of a triangular matrix and its conjugate transpose; in other words is a method to find the square root of a matrix.

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