We introduce a new hybrid analysis technique for large scale PEPA models that combines stochastic simulation with ODEs, providing a finer trade-off between accuracy of the solutions and the associated computational costs. We describe a version that employs ODEs for the mean component counts and also a version using ODEs for second order moments. This enables the calculation of covariances of component counts and also improves the accuracy of the mean approximations . We look at various examples demonstrating the advantages and limitations of this approach.
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