Last month, in Part Three of our ongoing discussion of making mistakes in the estimating process, I elaborated on the remaining three of six "elements" essential to controlling-if not eliminating entirely- mistake making. They were:
This month, we'll examine the types of mistakes made during the estimating process. A good place to start is the most basic element of performing a quantity survey: the "scale-in-use."
Trust but verifyA common mistake made by inexperienced (and for that matter, experienced) estimators involves the scale by which the drawings, whether they are plans, RCPs, elevations, sections, details, etc., are measured to determine size, area, length, thickness, etc. Take the case of the estimator who "made a good mistake."
The estimator who measured the plans and RCPs in 1/8 inch = 1 foot, 0 inches scale, closed the job and later discovered that the plans and RCPs were actually in 1/4 inch = 1 foot, 0 inches scale. Of course, as was discussed in a previous article, this "good" mistake is in reality a "bad" mistake simply because the situation could have been easily reversed. In all likelihood, the estimator used the scale indicated on the plan/RCP legend (usually in the title box near the drawing number) without verifying it to be the actual scale.
Often, a scale is indicated that is simply incorrect. Architects, like the rest of us, are falible human beings and such errors and omissions can occur-quite frequently it turns out-particularly with the advent of CAD. The computer's ability to manipulate data and images make such mistakes more commonplace than in the days-gone-by of hand drafting. It is contingent upon the estimator to verify that the scale-in-use is indeed correct. Most often, this can be easily achieved by measuring a "given" dimension, such as a masonry opening for perimeter windows.
It's always best to use a given dimension but of course these too can be inaccurate, but less likely so than the scale indicated. As a further check, measure a few other dimensioned lines and a door opening. Single-swing door openings are typically 3 feet, 0 inches wide. Whenever I work on a floor plan, RCP, etc., to determine quantities, I always find and circle the scale indicated and then follow the procedure outlined above to "trust but verify" that it is correct. To assume it is correct is to risk the fate of all assumptions; to make an ass out of you and me.
Sometimes, a scale simply is not indicated. Again, use dimensions and typical openings to verify and confirm the scale-in-use. Some scales can easily be mistaken for another. For example, 3/8 inch = 1 foot, 0 inches looks a lot like 1/4 inch = 1 foot, 0 inches to the naked eye. Likewise, 3/16 inch = 1 feet, 0 inches looks a lot like 1/8 inch = 1 foot, 0 inches.
Without checking and confirming the scale, measurement in the wrong scale can occur with serious consequences. This is why it is imperative that the first and most important task when performing a quantity survey is to locate and verify the scale-in-use. It's also important to use measuring tools that are in the same scale of the delineation to be measured.
For example, if a floor plan is in 1/16 inch = 1 foot, 0 inches scale, rather than measuring it in 1/8 inches = 1 foot, 0 inches and then converting the survey quantities to reflect the actual scale (1/16 inch) by doubling all the measured survey quantities, it is better to use a 1/16 inches = 1 foot, 0 inches scale from the outset. This eliminates the potential for an error/omission to occur when performing the conversion calculation. Truth be told, most errors in the estimating process occur when performing calculations. With modern computers, digitizers, electronic scales, etc., it is relatively easy to measure a delineation in almost any scale, even metric.
Another important aspect when dealing with scale measure is to take care when referring to other delineations. For example, an office building's overall floor plan may be in 1/8 inch = 1 inch, 0 inches scale while the "core" plans are typically elsewhere in the drawing set in 1/4 inch = 1 foot, 0 inches scale. Obviously, it's important to distinguish between these two scales. All the "shell" work such as column enclosures, window sills, heads, jambs, etc., will be quantified from the overall floor plan (1/8 inch) while the toilet/elevator "cores" will be quantified from the more detailed core plans (1/4 inch). When performing a quantity survey, it's always best to work from the largest, most detailed delineation. If this is not practical, use it to further define the delineation of the drawing by which you will determine quantities.
Hiding in plain sightLast, there's the plain old application of common sense when dealing with scale measure. I was once asked to review another estimator's quantity survey for content and accuracy. He seemed to have all his "ducks in a row." When I asked him what was the typical scale by which he measured the plans and RCPs he responded, "1/4 inch = 1 foot, 0 inches."
"Well, I think you have half the actual quantity required," I said.
"How could that be?" he indignantly responded. "I measured in the scale indicated on all the plans and RCPs which was uniformly 1/4 inch = 1 foot, 0 inches."
"OK, but these are half-size drawings from the printer. That means you would have had to measure in 1/8 inches = 1 foot, 0 inches scale (1/8 inch is half of 1/4 inch) to account for the fact that these drawings are one-half the size of the full-size drawing set which is in 1/4 inch = 1 foot, 0 inches scale. Otherwise, you will have to perform a conversion calculation (multiply by two) to adjust the survey to reflect this reality."
"Oh, I see," he responded.
Common sense: Don't leave home without it.
Next month, we'll continue our ongoing discussion of the kinds of mistakes made in the estimating process.