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Forecasting or Estimating of Construction Cost

"This chapter deals with general aspects of forecasting or estimating of construction cost. Definitions are first given. Roles of the estimators are described. Classes of estimates and accuracy are addressed."

Introduction
This chapter deals with general aspects of forecasting or estimating of construction cost. Definitions are first given. Roles of the estimators are described. Classes of estimates and accuracy are addressed. Next, the importance and preparation of pre-design estimating are explained. Following that, pre-design estimating models are reviewed in detail. They are grouped into three main categories: 1) empirical; 2) regression; and 3) probabilistic models. Factors affecting construction costs and causes of errors in estimating are also explained. Then, adjustment of the historical cost data, e.g. using cost indices is amended. Project budgeting is briefly reviewed. Finally, the roles of computer in estimating are discussed.

Definitions
Estimating may be defined as one or a combination of the following.
1) Estimating is an information processing approach to form construction cost estimate during the design process (Birrell, 1980).
2) Estimating is the compilation of all the costs of the elements of a project or effort included within an agreed upon project scope. To a contractor, this is the cost that will most likely be incurred to complete the project as defined in the contract documents and to turn it over to the owner (Nelson et al., 1982).
3) Estimating is a combination of a mechanical process and subjective expertise. It consists of application of appropriate unit rates to the measures finished quantities of a proposed building (Ashworth, 1988).
4) Estimating is the production of a statement of the approximate quantity of materials, time, and costs to perform construction decisions (Carr, 1989).
5) Estimating plays important roles in forecasting future events in construction process. It consists of two distinct tasks; 1) to determine the probable real cost; and 2) to determine the probable real time to build a project (Peurifoy and Oberlender, 1989).
6) Estimating is a technical function undertaken to access and predict the total cost of executing item(s) of work in a given time using all project information and resource (Kwaku, 1994).
7) Estimating is the fundamental process of the construction industry that answers the question "how much is the project expected to cost?" (Schuette and Liska, 1994).
8) Estimating the cost of building and civil engineering works takes into account the three major groups: 1) material, labour, and plant; 2) dayworks, provisional sums, and prime cost; and 3) preliminaries (Geddes, 1996).

As this research deals with pre-design cost estimating, estimating is defined as forecasting of building cost done at a stage when the design of building has not yet been performed. The process uses basic information of the building, e.g. height, structural system, foundation, exterior finishing, interior decoration, and site accessibility. This information is used in conjunction with price indices to account for changes in value of money. The term "estimating" and "forecasting" are used interchangeably in this dissertation.

Estimators
An estimator (or quantity surveyor, or cost engineer) is the person who prepares estimates (Schuette and Liska, 1994). In the planning, design, and perhaps, construction stages, an estimator is always involved for studies requiring thorough understanding of the principles and methods of engineering economics. He or she must often work closely with managers, accountants, financial analysts, and engineers to forecast the cash or borrowing needs for the project. As major decision is made from information contained in the conceptual or preliminary estimate, this places a responsibility and liability on the estimator. He or she will risk reputation when insufficiently accurate estimate is prepared for a bid but the owner or the contractor will risk money (ASCE, 1985; Schuette and Liska, 1994).

A good estimator must conceptualize the complete building before it is fully designed. He or she must be able to think, see holistically, and then perceive the details of the project (Carr, 1989; Raftery, 1991; Bledsoe, 1992; Schuette and Liska, 1994). The estimator must also have all the qualities and skills of a detailed estimator plus the ability to anticipate design decisions and communicate those assumptions made during the conceptual estimating process. He or she must also be knowledgeable of the expected life of construction materials, accounting, taxation, law, economics, and awareness of engineering design. Ahuja (1976) summarized qualifications for a good estimator which are: 1) patience of detail; 2) technical knowledge; 3) good memory; 4) knowledge of construction process; 5) able to plan the works; 6) have an idea of relative costs; and 7) good judgement.

Good estimator should not select information simply for its convenience or its appearance of objectively, e.g. using cost data from projects or published estimating guides without sufficient knowledge (Carr, 1989). Consequently, he or she must not spend so much time and effort to analyze unnecessary details too precise in determining the costs of insignificant items as the estimating will take time and be expensive (Carr, 1989; Diamant and Tumblin, 1990). In a bill of quantities for civil engineering project, 80% of the costs can be attributed to 20% of the items, and vice versa (Ashworth, 1988; Poh and Horner, 1995).

The estimator who has many years of experience in estimating is called an expert. Based on very little project information, the expert can exhibit expertise in performing cost estimating which are: 1) more relax but confident; 2) more concerned with maintaining familiarity with the market and overall price levels than others who are more concerned with careful analysis: and 3) able to recall the overall price of the projects undertaken (O'brien, 1994). Skitmore and Wilcock (1994) found that smaller builders price only half of the items in bill of quantities for competitive tender by the detailed methods prescribed in the standard texts. The remaining items are rated mainly by experience. The more experienced the estimator, the more the likelihood that subjective decisions will be based on objective experience and personal bias (Bladey et al., 1990).

Classes of Estimates
Estimates can be categorized into several classes according to purposes, budget limitation, time, and accuracy (Antill, 1970; ASCE, 1985; Peurifoy and Oberlender, 1989).

Antill (1970) classified three basic types of estimates. First, approximate estimates (or rough, or pre-design estimate) is generally applied to assess the magnitude of a project prior to its design. It can also be performed with minimum preparation of design and related information. Therefore, it is based on a sound intelligent guess, judgement and experience of similar projects. Second, preliminary or definitive estimate is used for accessing the relative costs of alternative designs or comparing alternative schemes in economic studies. It needs designs and preparation of drawings. When more details and a reasonable quantity can be obtained, the accuracy can possibly be lowered. Third, detailed estimate provides the final assessment of the cost of a project. It is compiled from; 1) drawings; 2) specification; and 3) bill of quantity. Ranges of accuracy are depended on the methods of obtaining the costs, i.e. ฑ8 to ฑ12% for unit rates, and ฑ5 to ฑ8% for basic cost method.

The AACE (1979) classified three classes of estimates; 1) order of magnitude; 2) budget; and 3) definitive estimates. The order of magnitude can be made any detail but it is often developed by using cost-capacity curves, scale-up or down factors or ratio estimating techniques. The budget estimate is referred to the owner's budget which needed flow sheets, layouts and equipment details. The definitive estimate is developed from very defined engineering data including plans, elevations, system and instrument diagrams, equipment data and quotations, soil data and major foundation, and completed specifications. The AACE stated general fact that estimates are more likely to be low than high due to numerous factors, including inflationary trends and incomplete data among others.

Nelson et al. (1983) grouped cost estimates into two main categories; 1) preliminary estimate; and 2) detailed estimate. The former can further be divided into two sub-categories; 1) order of magnitude; and 2) conceptual estimate. The latter could be divided into three sub-categories which were; 1) semi-definitive estimate; 2) definitive; and 3) check estimate

Bowen and Edwards (1985) verified the historical records obtained from the architects, and quantity surveyors. They classified error margins in estimating according to the stages of works: 1) conceptual or cost target; 2) sketch design; 3) design investigation; 4) drawings submission; and 5) bill rate.

Hendrickson and Tung (1989) classified cost estimates into three main groups. First, the design cost estimate is used for the owner or designers. It runs parallel with the planning and design, and can further be divided into four subgroups: 1) screening estimate (or order of magnitude estimate); 2) preliminary estimate (or conceptual estimate); 3) detailed estimate (or definitive estimates); and 4) engineering's estimate. Second, the bid estimate consists of direct cost plus markup, the direct cost is generally derived from: 1) sub-contractor quotations; 2) quantity takeoffs; and 3) construction procedures. Third, the control estimate is used for monitoring the project during construction. It can be derived from available information to establish: 1) budget estimate for financing; 2) budgeted cost after contracting but prior to construction; and 3) estimate cost to completion during the progress of construction.

Peurifoy and Oberlender (1989) divided cost estimates into two classes: 1) approximate (or preliminary or conceptual or budget) estimates; and 2) detailed (or final or definitive) estimates. They explained that estimating consists of two main tasks; 1) to determine the probable real cost; and 2) to determine probable real time to build a project.

Ogunlana (1989) summarized the nature and characteristics of estimating which are: 1) accuracy improves with the development of the project such that the distribution of errors narrows from feasibility to settlement; 2) underestimates are more likely than overestimates; and 3) the final cost of a project cannot be established until the settlement of project accounts.

Barrie and Paulson (1992) classified cost estimates into three classes. First, conceptual or preliminary estimate is used for budgeting purposes. It is prepared to prior bidding. Second, fair-cost estimate is normally prepared by the contractor based on the bid documents. It is used for bidding. Third, definitive estimate is prepared to fix anticipated cost of the project within little margin of error.

Poh and Horner (1995) stated that reduction in the number of items for which prices are estimated might encourage an improvement in accuracy of the unit rates derived by the estimator. So, the total accuracy of cost-significant models might not be very different from the values of 5 to 15%, typically achieved at the tender stage.

Pre-design Cost Estimating

1. Other names of pre-design estimating
Pre-design estimate means estimating done at early stage when full design of the project has not yet been completed. The purposes of pre-design estimate are to make a final decision on investment, budgeting, and financing the project. Pre-design estimate may involve several iterations of estimate in parallel with conceptual or preliminary design. The pre-design estimate can be called by other names, i.e. rough estimate (Antill, 1970), preliminary estimate (Antill, 1970; Neilson et al., 1983; Peurifoy and Oberlender, 1989; Barrie and Paulson, 1992; Bledsoe, 1992), Order of magnitude estimates (AACE, 1979), approximate (Antill, 1970; Peurifoy and Oberlender, 1989), budget estimate (AACE, 1979; peurifoy and Oberlender, 1989), conceptual or cost target estimate (Bowen and Edwards, 1985; Peurifoy and Oberlender, 1989; Barrie and Paulson, 1992; Bledsoe, 1992; O'brien, 1994; Schuette and Liska, 1994; Elazouni et al., 1997), and feasibility estimate (Schuette and Liska, 1994).

2. Importance of pre-design estimates
Construction projects always start with the preparation of estimates of probable cost for budget, financing, bidding, accounting, and cost control (Kouskoulas and Koehn, 1974; Rurkpuritat, 1978; ASCE, 1985). It provides the frameworks for evaluating different alternatives at the conceptual design stage (Elazouni et al., 1997). Preliminary estimating is important for projects that utilize scarce resources because of limited funding and rapid increase of investment costs to develop the project (Sanders et al., 1992). Vergara and Boyer (1974) explained the processes of estimates. Before any civil engineering project starts, estimates need to be prepared. The owner has to engage the services of a designer to design a building that satisfies his or her desires within budget constraints. Thus, continuous estimates have to be made to inform the owner, or simply to assure an economical design. The estimates are very crude at beginning stage as they are prepared on the basis of rough designs. Using these estimates, continuous modification of the design is also made until the objectives are met. In preparing an estimate, a number of assumptions are required such that no method might be considered perfect. Since the process of accepting or rejecting the various proposal is based on the estimated costs, they must be as accurate as possible so that the results and conclusion will be as realistic as practicable (Neilson et al., 1983; Ashworth, 1988).

Barrie and Paulson (1992) explained that the preliminary estimate assists the overall cost-control program by serving as the first check against the budget. It will indicate the cost overruns early enough for the project team to review the design for possible alternates. Since preliminary estimate is made prior to the completion of detailed design, the margin of error will be relatively large. Then, the larger contingency should be applied. The contingency varies with the amount of design information available and the extent of cost information obtainable from similar projects.

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