Errors in Estimating (and How to Avoid Them)

Charles Mahaffey

December 2005

No matter how good you are as an estimator, at some point you will probably experience that sick, gut-wrenching feeling that comes with the discovery of an error in your estimate. These errors and the ways errors in estimates can be made are almost without limitation. Some of the different types of errors I have seen are these:

Error in judgment. In pricing the labor for a condition, you might not fully realize the difficulty of the condition, so you underestimate the number of hours required.

Error of omission or duplication. Failing to takeoff part of the project, or duplicating an area, results in underestimating or overestimating the cost.

Error in computation. When performing mathematical calculations by hand instead of using a computer, the opportunity for an error in computation is huge.

Entry error. Even when using a computer, the computer only knows to calculate what you have entered; it’s the human who makes the keystroke error.

Error scaling plans. This is big! I have probably heard about this error more than any other. It’s bad enough to use the wrong scale when we are not paying attention, but many times the architect will note the wrong scale on the plans, and the chance for a scale error is dramatically increased.

Error in scope. You might have the most accurate takeoff and estimate possible and then not have your scope correctly spelled out on your proposal. If the specifications for your section call for items that you do not intend to include, then don’t forget to exclude them in your proposal.

Keep errors to a minimum by being methodical. Develop a ritual and follow it with every takeoff and estimate you prepare. I always begin a project takeoff by reading the specifications, making notes and highlighting the parts of the specifications that contain the most relevant information. Next, I thumb through each page of the drawings and flag areas or details that are important or might be overlooked. I also make note of the deck heights and get a general feel for the project.

When I am performing a takeoff on paper plans, I carefully takeoff the condition and then color what I have just taken off. I have seen estimators color the condition first and then take off the condition. This is asking for trouble—I don’t know how you would keep up with what you have done, especially if you are interrupted in some way.

Using takeoff software will significantly reduce your chance for takeoff errors because as you are performing a takeoff, the software is coloring and quantifying for you. You eliminate the possibility of forgetting where you left off and you also eliminate the possibility of writing down the wrong number. The use of this type of software is the single most important thing you can do to minimize takeoff errors. Remember that it is the human element that is responsible for the error, so removing the opportunity for human error from the estimating process will maximize the accuracy of your estimate.

Performing a double-check of all calculations and entries is a must. If you work alone, you will have to check yourself, but if you work in an office, it is a good idea to have someone else check all of your work.

After the estimate is complete, you must then perform comparisons. You can compare the square foot or linear foot cost of similar conditions. Also compare types of previous jobs (hotel, high-rise, etc.) by the cost per square foot of the entire job as well as labor-to-material ratios.

Develop (and stay with) a ritual for your takeoff and estimating. Analyzing the project as I have recommended and maintaining a disciplined approach will help minimize your estimating errors and might save you from that gut-wrenching feeling.

About the Author
Charles Mahaffey is president of Accuest, LLC, Marietta, Ga. Accuest provides estimating and consulting services for commercial drywall subcontractors.