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We're often complimented on the
2 1/2% accuracy rate of our estimates. In this month's featured article are a few pointers towards achieving accuracy in your own estimating. |
Accuracy In Estimating
The keys to accuracy in cost estimating are understanding the building you are pricing, making detailed measurements of the component systems, and then pricing them correctly.
Understand the Building
The first key is to understand how the building you are pricing will be constructed. Only when you fully understand the building and its component systems can you measure them correctly. It's relatively easy to do this at the construction documents stage, when everything is detailed on the drawings, but not so at concepts or schematics. At these early design stages it is critical to know what is not yet shown, and to estimate those components too. Before you can build your estimate you must be able to build your building, from foundations to rooftop, in your mind's eye.
Accurate Measurement is Critical
The potential for making gross errors in an estimate is far greater during measurement than in pricing. Prices change rapidly but seldom by more than a few percentage points while mismeasurement can cause errors of far greater magnitude. Some common measurement errors to look for in reviewing an estimate are:
i) Incorrect Scales. The scale shown on drawings is sometimes incorrect and must always be checked. For example, a ¼" scale drawing measured to a 1/8" scale generates lengths that are 100% out and areas 300% too large.
ii) Mismatched Units. Another common error is to take measurements that do not match the pricing source. For example, a parking lot measured in square feet but then priced at a square yard rate generates an error of 800%.
iii) Missing Items. This comes back to understanding the building. Any item missing from the estimate is 100% out.
How Far Can You Go Wrong?
Huge mistakes can easily be made be when an estimate is rushed. One estimate we reviewed, that was prepared by a well known Construction Management firm, contained $10 million in mathematical measurement errors and very little in pricing errors.
The simplest of things can cause quite large errors. In comparing our estimate for a University Science Building with another well known Construction Management firm's estimate we were initially puzzled by a $2 million difference, even though quantities and prices were very closely reconciled. A mathematical check uncovered the reason. At every step in the CM's estimating program costs were rounded up and carried forward to a series of other calculations and summaries where they were rounded up again and again. The cumulative result of all these rounding errors was to inflate their estimate by $2 million.
A More Accurate Methodology
Be sure to measure in the same units that your pricing sources use.
Measure all areas and lengths as accurately as possible.
Use an orderly take-off methodology and a detailed checklist. Be sure nothing is missed.
Take the time to do it right. A rushed estimate has the most potential for gross errors.
Always check the math. Never have blind faith that the computer is right. The calculations programmed into a spreadsheet or estimating program may not be the calculations you expect.
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