Opinions differ on whether it's worth the time during the estimating stage of a project to compile a detailed materials list. If a job is estimated with the expectation of doing the work, then a rational materials list will save a second estimating process. Therefore, after transferring information from the takeoff forms, the estimator's next task is to account for all the nomenclature of the job. Using assembly units makes the process a painless one. When assembly units are used in computer systems, they save time and ensure accuracy. But for those just beginning their estimating careers, the elementary process of "exploding" a plan symbol is still the best foundation for understanding what the computer's abilities and limits are. Errors are likely when item quantities are described by differing terms. If a length of conduit is entered in the material column with a base quantity of "C," meaning 100, and the labor is entered as per foot, there is a distinct possibility of a mistake. The symbols or abbreviations used are "Ea" that stand for each, or a single count. If materials are priced in lots of 100, then the applied symbol is the Roman numeral "C." Most wire is priced by the 1,000 feet, which is represented by the Roman letter "M." The quantity descriptor must be the same in the material and labor columns to avoid errors. A comparison of the NECA Manual of Labor Units and pricing services may indicate that quantity descriptors vary at times. For example, in the case of rigid steel elbows, "E" is shown in the manual as being labored per each, while in the pricing reporting services, the price for that elbow is shown as being per 100, or "C." Since the same quantity multiplier is used for both labor hours and material costs, it becomes important that the designator for material match that of the labor. This matching of descriptors will ensure that when the manual extension is done, the extension will be accurate and speedy. The listing process must be complete in every detail, either by using an assembly unit or a breakdown of all items needed. This detailed listing of individual items or assemblies will ensure the application of appropriate labor units. When entering labor hours, minutes must be converted to a decimal number. The NECA Manual of Labor Units includes a typical conversion chart based on minutes. A simple formula, such as dividing the number of minutes by the 60, will produce the same desired decimal. Adding minutes and hours requires distinct math processes; converting time to decimals will avoid errors and speed up manual extension calculations. For profitable projects, realistic labor units must be used. Many estimators yearn for a prescriptive set of labor units that will solve all ambiguities similar to those used to prepare estimates for car repairs. Assigning labor units is the part of the estimate where experience and past company history come into play. The typical published labor unit has many variances built into it, which must be understood before placing blind faith in such units. Estimators, through long experience and much data, have managed to arrive at a set of labor tables on which they rely. Such labor tables, as well as those developed by individual companies, are proprietary and are rightfully guarded to avoid frivolous use or disclosure. NECA's labor units originated in the early 1920s. With appropriate factoring, these units have been proven, and continue to be revised and proven over many years. No matter which labor units are used, estimators are cautioned that they are solely responsible for the selection of applicable units, a task that cannot be taken lightly, nor performed by the inexperienced. There are great differences among labor tables. The blind use of any readily available units could be disastrous for the contractor. It is imperative that before any labor unit listing is used that the scope of the labor unit is closely examined. Each section of the NECA Manual of Labor Units included a scope of work and a listing of items to consider before completing the bid. Without a like comparison of all available labor tables the use of such tables becomes a gamble. One factors affecting how much time a specific task requires is the quantity of a particular operation is to be done at a given time. It is obvious that the installation of one fixture will take much longer than would ten fixtures. Some customers will not understand this situation. To assess the amount of time required for such installations, a task analysis should be done, including all the operations required getting the fixture installed. It is not a case of the fixture magically appearing at the installation site; many operations precede such an installation. If the quantity increases, then many of the operations will be done for ten fixtures instead of one, save the actual installation of the fixture to the building. NECA's Manual of Labor Units, as an example uses a base of a building that is 20,000 to 100,000 square feet in size, with fixture quantities and devices that match buildings of such size. Awarding authorities may require or prohibit that change orders use a specific set of labor tables. Estimators knowledgeable on the subject of labor tables should professionally resist requests or orders of such a nature. For example, there was a project in a state where the federal authorities required that a specific publisher's labor units be used. The irony of the order was that, in his foreword, the publisher warned the user that the labor tables should not be used in that state as well as another, more than likely due to their geographic locations. Most estimating is done by computer. Many contractors turned to computers early in the game, only to find out that the labor units that came with some programs were fictional. Before blindly using estimating programs, scrutinize their labor units, and determine the depth of the assembly units and whether those labor units can be customized. It is no different with manual estimating practices, where the estimators can custom fit the labor units to what expectations of the project will be. DAVID is a professor of electrical technology at Long Beach City College, Calif., a consultant, and an expert witness. He can be reached at (562) 597-1877or by e-mail at firstname.lastname@example.org.