Risk results from uncertainties in realized

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Unformatted text preview: ctiveness from the fact that it was a "first"; a mission to Mars that misses its launch window has to wait about two years for another opportunity—a clear schedule constraint. Risk results from uncertainties in realized effectiveness, costs, timeliness, and budgets. Sometimes, the systems that provide the highest ratio of effectiveness to cost are the most desirable. How The System Engineer's Dilemma At each cost-effective solution: • To reduce cost at constant risk, performance must be reduced. • To reduce risk at constant cost, performance must be reduced. • To reduce cost at constant performance, higher risks must be accepted. • To reduce risk at constant performance, higher costs must be accepted. In this context, time in the schedule is often a critical resource, so that schedule behaves like a kind of cost. ever, this ratio is likely to be meaningless or—worse— misleading. To be useful and meaningful, that ratio must be uniquely determined and independent of the system cost. Further, there must be but a single measure of effectiveness and a single measure of cost. If the numerical values of those metrics are obscured by probability distributions, the ratios become uncertain as well; then any usefulness the simple, single ratio of two numbers might have had disappears. In some contexts, it is appropriate to seek the most effectiveness possible within a fixed budget; in other contexts, it is more appropriate to seek the least cost possible with specified effectiveness. In these cases, there is the question of what level of effectiveness to specify or of what level of costs to fix. In practice, these may be mandated in the form of performance or cost requirements; it then becomes appropriate to ask whether a slight relaxation of requirements could produce a significantly cheaper system or whether a few more resources could produce a significantly more effective system. Usually, the system manager must choose among designs that differ in terms of...
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