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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
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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