Designing Complex Systems

40 n Designing Complex Systems

does not necessarily have to be linear). Second, the QoS should be a single-valued

function of its defining parameters. And third, even though the parameter values

might in practice only vary over a limited range, the QoS should show a reason-

able behavior for limiting values of the parameters. For example, in the case of the

Mine, if either the nominal capacity or the availability go to zero, the QoS should

go to zero, so that the combination is that of a product rather than a sum of the

two parameters.

We recognize that expressing the performance of a possibly very complex sys-

tem by a single parameter is a great simplification, and that the purpose of doing

so is simply as part of creating a starting point for ...