time points (obtained with the SSA), repeated three times, resulting in nine data points. Also when looking at the distance form these nine points from the deterministic mean, it is clear that for the dimerisation system $(x-\mu)$ is small, whereas for the p53 system it is [relatively]{} large, indicating that a larger number of moments is necessary to capture the full distribution.

Such simple heuristics have the advantage of being computationally affordable. While inadequate at guaranteeing good performance of an expansion using any finite number of moments, we can use them to capture any gross inadequacy of a given approximation relatively reliably. Such small-scale analyses should precede or accompany moment expansions. More generally, we can consider this problem from the point of view of statistical model checking; see e.g. [@Gelman:2003]. But the question as to how many stochastic simulations need to be averaged over to get a good idea of the mean (or any higher moment) is challenging to answer for all but the most trivial systems [@Toni:2008aa].

![\[\]Study of the deviation from the mean $(x-\mu)$ (a) Deterministic mean and single SSA trajectory for dimerisation system. (b) Deterministic mean and 9 points taken from different SSA trajectories for dimerisation system. 