OK, thanks for the answer. And in terms of practical use, how is the figure interpreted? What does it actually mean? How can I use it to improve the performance of my systems?

I'm not sure what the figure is actually telling me, but if I got a prediction error of 18%, does that mean that my performance statistics actually lie somewhere within +/- 18% of the quoted values? Shouldn't the prediction error then be accompanied by some sort of confidence interval?