Project Cost Estimation Based on Standard Price of Goods and Services (SHBJ)

Cost estimation is determining the probable cost of a project, which is critical in project development. For government projects in Indonesian construction industry, it is common to use a standard price of goods and services (called SHBJ) publised by the government as a cost reference. This research examines whether the SHBJ provide sufficient and reliable data. A case study from a contractor’s bidding document (RAB) and the SHBJ from the city of Yogyakarta are employed. The result shows that cost estimation based on the SHBJ and the contractor’s RAB gives 12% difference. Using SHBJ data from years 2010 – 2016 a regression equation is obtained as Yi = 62,156 X + 2,003,962 which can be used to estimate project cost in the future. This research also suggests some improvments for the SHBJ.


Introduction
Cost estimation is determining the probable cost of a project, which is critical in project development. Cost estimation is needed by all parties of project such as owner who to obtain reasonable price; engineer to develop enginner's estimate and contractor to bid a project.
There are several methods of cost estimation such as expert judgment, analogous estimate, parametric estimate, detailed estimate, etc. [1]. Expert judgment is determining cost estimates based on expert's experience from the past. Analogous estimate calculates based on similar projects. Parametric estimate applies parameter such as area per square meter. While detailed estimate calculate based on detail components of the project. Each method has its own certain level of accuracy, feasibility estimate 25-30%, appropriation 15-25% and detailed estimate 5 -15% [2]. Cost accuracy is not easy to determine, but it can be detected from large cost overrun or underrun [3].
In construction projects parametric and detail estimates are the most commonly used methods. Here is an example of parametric estimate for a building with area of 1.000 m2 and cost per square meter Rp. 5.000.000; so the estimated cost will be 1,000 m2 x Rp. 5,000,000 = Rp. 5.000.000.000. Whereas in detail estimate the calculation is based on the volumes of all components of the project multiplied by the unit prices. So cost will be equal to volume times the unit price. The unit prices are determined from a standard unit price and depend on the cost of resources (materials, wages, equipment, etc.).
There several sources of data as cost references such as catalogs, surveys, historical data, etc. Reliable data is very important since inaccurate data can cause loss [4]. Standar price of goods and services (SHBJ or Standar Harga Barang dan Jasa) published by the government annually is widely used. The prices in the SHBJ are the highest price limit allowed in a government project. For construction sector the SHBJ provide prices of materials and wages and the unit price per square meter for building projects [5].
This research aims to examine data of the SHBJ, i.e. the price of materials and wages and the unit price per square meter, whether the SHBJ has accomodated all data required for developing cost estimate and whether they provide fair prices. This research also utilises SHBJ from 2010 to 2016 to obtain regression equation for cost prediction. Lastly this research suggests improvment of the SHBJ.

Methodology
This research uses a case study from a contractor's bidding document (called RAB) which is a detailed estimate. The RAB consists of several tables, i.e. Recapitulation, Bill of Quantity, Unit Price Analysis and Price of Material and Wages. Calculations in the RAB basically following steps as shown in Figure 1 and can be explained as follows: • Detemine the price of material dan wages • Calculate the unit price, usually using SNI standard • Calculate the bill of quantity • Cost recapitulation

Fig. 1. Steps in cost calculation
To calculate cost estimate based on the SHBJ, data from the SHBJ are inputted into the table of Price of Material and Wages. If data are not available in the SHBJ then data from the contractor's bidding is applied. The contractor's bidding is in 2015, so to obtain prices for different years, annual indexes of construction prices are used. Table 1

shows construction indexes published by the Central Bureau of Statistics. Price of material in year i is calculated as follow:
Price For example, a material iron strip is not found in the SHBJ 2010 and the contractor's price is Rp. 12.000. Then the price in 2010 is calculated as (100 / 130,27) x Rp. 12,000 = Rp. 9.211.

Project in the Case Study
The case study project is a two-story office building with an area of 1,258 m2 located in Gunung Kidul distric, Yogyakarta. The building is a reinforced concrete structure with lightweight steel roof truss. The components of the building consist of 14 groups of work, some of which are preparation, earthwork, masonry, concrete, roof works, etc. The RAB is Rp. 2,743,289,872 before tax and it is in 2015.  Table 2 shows the SHBJ's cost per square meter in 2010 -2016. By multiplying these costs with the area of the buildings, costs of the building for these years can be obtained.   Some of material prices in the SHBJ have significant difference from the contractor's prices such as wood, coating and alcasite which have difference up to 24 -36% and other materials even have difference greater more than 200% such as ridge, tap, floor and nail. The prices of these materials need to be rechecked.

Project Costs Based on the SHBJ
Some materials have different names on the SHBJ compare to that in the contractor's document such as for sandpaper, door lock, gravel, common rafter, nail, etc. Although these differences can be understood, but standardization of names would be beneficial for clarity. Similarly some materials have different units such as meter vs piece, m2 vs sheets, m3 vs piece, etc. Standardization of units is also important for ease and to avoid errors in cost calculation.

Conclusions
The SHBJ provides two references for cost estimasting, i.e. the price of materials and wages and the standar price per square meter to estimate building projects. The price of materials and wages in the SHBJ provides quite accurate result as can be seen from the case study that it results in difference of about 12% against the contractor's RAB. Whereas the standar price per square meter gives quite a large difference of up to 62% -126%. The standar price per square meter therefore need to examined further to provide reasonable price. Based on the analysis of annual prices from the SHBJ in 2010-2016, a linear regression equation is obtained, i.e. Yi = 62,156 X + 2.003.962 which can be used to estimate cost per square meter in the future. The equation can be applied for projects in Gunung Kidul District of Yogyakarta, but for other places should be adjusted with location factor. The study also suggest some improvement for the SHBJ such as to compelete resources which are not available yet, more accurate price, and standardization of resource's names and units.