the expansion at a low order will only result in a good approximation in case $x$ is close to point $c$, where we have approximated the true function. In our case $c$ is the mean, $\mu$, implying that an approximation using a few moments will be accurate only in case all observations are close to the mean. In case it is possible to perform a single realisation of the SSA, we can assess this quality by comparing the mean calculated with the deterministic approach with the trajectory calculated with the SSA (Figure \[p53figure5\]).

Figure \[p53figure5\]a displays the deterministic mean of $x_1$ and one SSA simulation for the dimerisation system. In this case the trajectory is close to the mean over the complete time course. A single trajectory for p53, displayed in Figure \[p53figure5\]c, compared to the deterministic result shows that in this case the distance of the trajectory from the mean is much larger. In case a single SSA realisation of the model is not possible, but experimental data are available, the distance form the mean can still be investigated in the same way. Figures \[p53figure5\]b,d show the mean calculated using the deterministic approach together with ‘measured’ values at three 