Project Cost Estimation Using a Stepwise Approach: a Case Study of an Infrastructure Project in Gresik Regency, East Java

. There are many studies related to cost estimation applied in a project, but only a few employ using “stepwise” method. This approach is expected to assist in solving complex problems by establishing a criteria hierarchy, subjectively assessed by interested parties, and incorporating various considerations to develop weights or priorities. The foundation of the stepwise approach relies on well-documented findings, which assert that 80% of the total project cost corresponds to the 20% most expensive components of the work. The research systematically collected data for project development estimates based on the bill of quantities from each year, as well as the HSPK (activity main unit price) in Gresik Regency from 2016 to 2018. This research aims to develop a conceptualized estimation model that provides clear, rapid, and relatively accurate initial project cost information. The results of this research include analysis of stepwise approach on cost realization, analysis of accuracy of the cost estimation model for buildings using the stepwise method, and an analysis of the cost of road and bridge infrastructure projects to facilitate completion in Gresik Regency. The research concludes that the analysis of the Cost Significant Model's impact on cost realization in the Public Works and Spatial Office's Highways Division in Gresik Regency has resulted in a cost estimation model for road development with an accuracy range from -12.7% to 16.0%, with an average accuracy rate of 0.59%. Additionally, there is a very strong correlation (R) of the independent variable (work in Division 2) and the total development cost, indicating that fluctuations in the cost of significant components are closely linked to changes in the overall development cost. The analysis of infrastructure project costs related to roads and bridges for project completion revealed that the highest value among independent variables is associated with Division 7, at Rp. 4,403,054 per m2. This suggests that the cost of Division 7 work is the most influential, while Division 1 (X1) has the lowest cost, at Rp. 35,589 per m2. Furthermore, there is a work component with zero cost, namely, Division 4 works.


INTRODUCTION
Related to cost estimate, there are already many studies applied in a project, but there is a few applying using "stepwise" method.By applying the method, it is expected to assist for solving complex problems by composing a criteria hierarchy, it is assessed subjectively by interested parties then drawing various considerations to develop weights or priority [1] The foundation of stepwise approach is relying on the well -documented findings which assert that 80% of the total project cost is attributed to the 20% the most expensive work items.The research data were systematically collected for project development estimates, based on the bill of quantities for each year, as well as the HSPK (activity main unit price) in Gresik Regency from 2016 to 2018 [2].
Gresik Regency is a regency located on the north coast of East Java and is characterized by mostly low-lying coastal areas.This landscape is an extension of the North Kapur Mountains.In the central region, there are lowlands and undulating areas, with some areas being marshy.The eastern edge of the Kendeng Mountains also extends into this central area.
Selection of the foundation type for retaining walls is a crucial step in building planning.The stability of slopes can be compromised by various human activities and natural conditions.Unstable slopes pose significant risks to their surrounding environments, highlighting the need for slope stability analysis [3].
Department of Public Works and Spatial Gresik Regency, particularly Division of Highways, employs several methods for cost estimation based on project planning stages , such as Engineering Estimate (EE) and Owner Estimate (OE).During the initial project planning phase, owners or principals must perform cost estimation to prepare the project budget.Therefore, there is a need for an easily applicable, accurate, and instantly accessible cost estimation model [4].Cost planning plays a critical role in the construction planning process, and the implementation of construction requires detailed analysis and documentation since cost estimation significantly impacts project success.The cost development model used is based on the principles of the Cost Significant Model Concept [5].
From a contractor's perspective, an ideal conceptual estimation model should exhibit certain characteristics.It should be simple, reasonably accurate, capable of providing instant feedback, consist of easily measurable elements, and effectively describe fieldwork operations.Such a model can serve as a reference for work monitoring and implementation [6].
Cost estimating serves as an important factor in the construction plan process in the context of Division of Highways, Department of Public Works, and Spatial Gresik Regency [7].It requires a comprehensive review during the development phase before the physical implementation of construction.Cost estimation significantly impacts the success of both the project and the company [8].
Stepwise is one of the construction total cost estimation models based on past bidding data.It places significant reliance on the prices that have the most influence on the total project cost.This reliance forms the foundation for estimation, which is translated into a multiple regression formulation [9].Stepwise Conceptual is a method for comparing cost estimations obtained from previous data.It involves determining the volume unit, such as area in square meters (m²) or per cubic meter (per-m³) for the building.Subsequently, the overall estimated cost is adjusted for inflation in the current year of the project's implementation.The unit price of work in each construction year is then calculated and serves as the price reference for subsequent pricing considerations [10].
This study aims to develop a conceptual estimation model that can provide clear, immediate, and easily accessible initial project cost information with reasonably accurate results.This study includes an analysis of the factors involved in the application of the stepwise method to cost realization, an assessment of the model's accuracy in estimating the cost of building projects using the stepwise approach, and an analysis of the costs associated with road and bridge infrastructure projects for project completion in Gresik Regency.

MATERIAL AND METHODS
Principally, the Cost Significant Model (CSM) is one of the total cost estimation models for a project based on historical cost data, and it relies on the most significant prices to influence the total project costs as a foundation for predicting estimates [11].It is essential for identifying project costs across different stages: (1) Conceptual development stage, where project costs are calculated globally based on a specific per-capacity price, (2) Construction design stage, where project costs are detailed based on work volume and unit price information, (3) Tender stage, where project costs are calculated by multiple contractors to secure the best offer, relying on comprehensive technical specifications and work drawings to establish a work contract, and (4) Implementation stages, where project costs are calculated with higher precision, taking into account the quantity of work, shop drawings, and implementation methods [12].
The definition of cost estimation, according to the National Estimating Society USA, is as follows: 'Cost estimation is the art of approximating the potential total cost of an activity based on available information at the current time [13].Cost estimation is closely related to cost analysis, which involves studying the costs of previous activities to serve as a reference for preparing cost estimates.In other words, preparing cost estimates is about anticipating, calculating, and making predictions about potential issues that may arise [14].Meanwhile, cost analysis focuses on the study and discussion of past activity costs to provide recommendations [15].
The 'conceptual Cost Significant Model' method involves several steps based on the analysis of previous project data.This includes excluding work items that, while sometimes numerous, do not have distinct work item classifications.Work item integration can be applied when the work items have similar unit sizes [16].The primary requirement is for the estimator to identify any essential considerations in a bid or the engineering approach needed to meet the project requirements.Developing a cost model is necessary to facilitate quick calculations, enhance project comprehension, and effectively communicate complex concepts [17].
Cost estimates are contingent on the progress of the project implementation timeline.Consequently, stages are established based on the detailed project development phases.[18]: 1) Preliminary estimation: it is made at the initial stage of the project to approach the economic feasibility of the project.It is also used for controlling financing and in price competition.2) Detailed estimation: it is made based on calculating work volume, costs, and detailed work unit prices.
3) Definitive estimation: it is a description of the financing and complete accountability for a project with only a small possibility of error.
In a construction project, the total cost of the project is the sum of the cost components which include labor costs, material costs, equipment costs, indirect costs, and profits which the percentage can be seen in Figure 1.

FIGURE 1. Sum of The Cost Components
Source : [19]

Cost Significant Model
The tender process in Indonesia is sometimes influenced by local culture.Contractors often provide only a brief identification and description of project requirements and engage in price negotiations.The Cost Significant Model is based on findings that are adequately documented, showing that 80% of the total project cost is associated with 20% of the most expensive work items [20].
The total value of a project is typically calculated by multiplying the total price of cost significant packages by appropriate factors, usually around 1.25.This gross value varies depending on the category and analysis of historical data.The planned work package reflects the field implementation [21], allowing for effective feedback and control.This method typically involves only 10% of the number of conventional budget items [22].
Simplifying this model reduces the time required for cost estimation compared to traditional cost budgets, which often consist of thousands of items.Cost Significant Models can achieve cost estimations within a 5% margin of accuracy, with the final calculation being even more precise, typically within a 1% margin.The accuracy can be improved or reduced by enhancing the model and relying on available data [23]." .

Project Cost Estimation
The use of project cost estimation by various professionals is outlined in Table 1.Estimation serves different purposes for different stakeholders [24]: 1) For owners, it is essential to evaluate project feasibility, assess the continuation of investments, determine the economic value of the project, and establish cash inflow and outflow requirements.
2) Planners rely on estimation to guide the design's implementation and ensure it aligns with project investment goals.It is crucial for planners to make informed choices about materials and project size that fit within the developer's budget constraints and meet the owner's satisfaction.3) Contractors use cost estimates to determine tender values and identify potential benefits, enabling them to execute the project as planned.4) Project managers rely on estimates to facilitate successful project completion by aligning with the budget set by the project planner.

RESULTS AND DISCUSSION
The data used as a research tool is the total cost budget, excluding Value Added Tax (VAT).The data can be seen in table 2 and Table 3.The research results will be arranged systematically based on the test results obtained from the results of the analysis and then linked with the expected results according to the predicted classification

Conclusion Finish
To create comparable project budgets, the researchers needed to standardize the project budget to a single fiscal year.In this context, the total project budget (Y) and its individual components (X1 to X8) were adjusted for the Division of Highways.Additionally, all data was further adjusted to account for the time value effect, which involved projecting the data to the year 2020.
The dataset includes detailed information for each work package, comprising total costs (Y) and associated work items, which are utilized as independent variables (X).The independent variable data are as follows:

Data Description
Data description is employed to present data in a quantitative form without involving decision-making processes.The data is presented descriptively, without undergoing further analytical techniques.Additionally, cost-significant items are identified as those comprising the largest portion, accounting for 80% or more of the total cost.These identified independent variables, representing the cost-significant items, are subsequently utilized in the analysis.The following table illustrates the percentage for each work component:

Determining Cost Significant Item
The three divisions of work in the previous sub-chapter, known as the cost significant items are then calculated as future values used as projections for 2020 costs (see Appendix 3).After that, the determination of cost-significant items will be analyzed using the SPSS program using a stepwise regression analysis technique, namely a method for determining the dominant independent variable.SPSS input data is as follows: , that has zero costs.. b.A Cost Significant Item is a summary of work costs that make up ≥ 80% of the total, including the works in Division 2, Division 5, and Division 7. c. Analysis using SPSS confirms that works in Division 2 (X2), Division 5 (X5), and Division 7 (X7) significantly affect the total construction cost.

TABLE 2 .
Project Budget of 2016-2019 Division of Highways, Gresik Regency

TABLE 4 .
Percentage of Each Work ComponentSource: Processed Data

Table 4
shows the percentage of each division of work as well as ranks starting from the division of work that has the largest cost to the cost with the smallest percentage of costs.It is known that there are three cost divisions with a cost percentage of more than 80%, as presented in Table5below:

TABLE 6 .
SPSS Rp.1,270,813.04.The difference between CSM and the actual HPS cost is nearly zero, with a CMF (Cost Model Factor) of 1.00000%.The average accuracy rate is very small, at 0.00%.3.The results of the cost analysis of road and bridge infrastructure projects to complete the project conclude that:a.The highest independent variable value is in the work of Division 7 (X7), which is Rp.4,403,054 per square meter.This illustrates that the work costs of Division 7 are the most dominant, while the work of Division 1 (X1) has the lowest cost, at Rp. 35,589 per square meter.Additionally, there is one work component, Division 4