Make a defensible engineering decision
A technically feasible design is not automatically the economically preferred design. Engineering economy compares consequences from a consistent viewpoint over a stated horizon.
| Alternative | Benefit ($) | Differential cost ($) | Net relative to A ($) |
|---|---|---|---|
| A | 9,000.00 | 0 | 0 |
| B | 12,000.00 | 2,000.00 | 1,000.00 |
B provides at least as much net benefit on this one-period monetary criterion.
Choose a consistent owner viewpoint and one study period.
Cash-flow dates and signs are part of the answer. Model values retain precision; displays round money to cents. Illustrative assumptions exclude taxes and inflation unless stated.
What the model means
Develop feasible alternatives, including doing nothing when optional. Focus on differences, use one decision-maker viewpoint, express comparable outcomes in common units, consider all relevant criteria, make uncertainty explicit, and revisit the decision after implementation. These seven principles prevent a polished calculation from answering the wrong question.
Build the calculation
A one-period comparison needs no discounting. With multiple dates, first move cash flows to a common date. Safety, capacity, reliability, and legal requirements may rule out an alternative before money is compared. A cartoon or surprising claim should prompt you to inspect incentives, omitted costs, and timing—not accept an apparent saving without a model.
Set up. Calculate. Explain.
Rebuild the starting scenario
Benefit of design A: 9000 $; Benefit of design B: 12000 $; Additional cost of B: 2000 $. Calculate the extra net benefit of B.
Setup hint
Choose a consistent owner viewpoint and one study period.
Open the visual solution
- Choose a consistent owner viewpoint and one study period.
- Common costs cancel; only benefit differences and additional cost remain.
- Compare 3,000.00 extra benefit with 2,000.00 extra cost. Also check safety and feasibility.
| Alternative | Benefit ($) | Differential cost ($) | Net relative to A ($) |
|---|---|---|---|
| A | 9,000.00 | 0 | 0 |
| B | 12,000.00 | 2,000.00 | 1,000.00 |
B provides at least as much net benefit on this one-period monetary criterion.
Choose a consistent owner viewpoint and one study period.
Cash-flow dates and signs are part of the answer. Model values retain precision; displays round money to cents. Illustrative assumptions exclude taxes and inflation unless stated.
Checks allow 0.1% numerical rounding (minimum $0.05 or 0.001 in the stated nonmonetary unit). Interpolated factors and rates are approximations; keep table precision consistent.
Transfer the idea to a new scenario
A provides $8,000 benefit; B provides $11,500 benefit and costs $4,000 more. Calculate B’s extra net benefit in the same period.
Setup hint
Choose a consistent owner viewpoint and one study period.
Open the visual solution
- Choose a consistent owner viewpoint and one study period.
- Common costs cancel; only benefit differences and additional cost remain.
- Compare 3,500.00 extra benefit with 4,000.00 extra cost. Also check safety and feasibility.
| Alternative | Benefit ($) | Differential cost ($) | Net relative to A ($) |
|---|---|---|---|
| A | 8,000.00 | 0 | 0 |
| B | 11,500.00 | 4,000.00 | -500.00 |
A provides the larger net benefit on this criterion.
Choose a consistent owner viewpoint and one study period.
Cash-flow dates and signs are part of the answer. Model values retain precision; displays round money to cents. Illustrative assumptions exclude taxes and inflation unless stated.
Checks allow 0.1% numerical rounding (minimum $0.05 or 0.001 in the stated nonmonetary unit). Interpolated factors and rates are approximations; keep table precision consistent.
Which comparison follows the principle of focusing on differences?
Can you defend your setup?
Can you place each amount on the timeline, match the interest period, calculate the equivalent quantity, and state the decision or limitation?
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