An order of magnitude estimate is prepared to an accuracy of roughly
Answer: A
It is made from capacity figures and past projects before any engineering has been done, so it serves only to decide whether an idea is worth pursuing further. Committing funds on such a figure is how projects come to overrun, since the estimate was never intended to bear that weight.
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A definitive estimate is distinguished from a preliminary one by resting on
Answer: B
Detailed drawings, bills of quantities and supplier prices bring the accuracy to within a few per cent, which is what a contract award requires. The work needed to produce it is itself substantial, so it is prepared only once the project is likely to proceed.
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The accuracy of an estimate improves chiefly as
Answer: C
An estimate can be no better than the information it rests on, and early in a project the scope itself is still uncertain. Adding contingency covers that uncertainty rather than removing it, and no amount of skill can price a design that has not been settled.
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The purpose of stating an estimate's accuracy class is to
Answer: D
A number quoted without its class invites the reader to treat a screening figure as a budget. Saying plus or minus forty per cent alongside it is what makes the estimate honest and useful, and it is the single most important convention in the subject.
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A parametric or cost estimating relationship expresses cost as a function of
Answer: A
Cost per megawatt, per square metre or per tonne of throughput is derived by regression from past projects and then applied to the new one. Its value depends entirely on the new project resembling those in the sample, which is a judgement the arithmetic cannot make.
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The estimate by analogy method prices a new project by
Answer: B
One comparable project is taken as the base and adjustments are made for differences in size, location, specification and date. It is quick and often surprisingly good, but it carries whatever was peculiar about that single base project into the new figure.
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A detailed or bottom-up estimate builds the total from
Answer: C
Every item is measured, priced and summed, which is the most accurate approach and by far the most laborious. It cannot be attempted until the design is essentially complete, which is why the earlier classes of estimate exist at all.
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The unit rate method estimates cost by multiplying a measured quantity by
Answer: D
Rates per cubic metre of concrete or per metre of pipe are held in a schedule and applied to the quantities taken off. It sits between analogy and full detail in both effort and accuracy, and is the workhorse of construction estimating.
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The six-tenths rule scales cost with capacity by raising the capacity ratio to a power of about
Answer: A
Doubling the capacity of a plant multiplies its cost by two to the power 0.6, that is about 1.52, rather than by two. The economy arises because vessel and pipe cost grows roughly with surface area while capacity grows with volume.
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The six-tenths rule reflects an economy of scale because cost rises
Answer: B
An exponent below one means the cost per unit of capacity falls as the plant gets bigger, which is why process industries build large. The exponent is not universal and must be taken from data for the particular kind of equipment.
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