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Ideally all cost drivers for a product needs to be identified identified, but in practice the number of drivers is usually limited to those that have the most significant impact on cost. Thus there is always a limitation that there may be more cost drivers which may have been missed in the earlier stages of estimation 
Point estimation in simple ABC models does not provide information on the sensitivity of product costs to process variation.
The integration between cost information and time in this method is not easy.
Also the cost incurred in real time application of this model is on the higher side.The value this model provides for the cost it incurs will be a determinant factor in its usage in the organisations especially medium and smaller ones.
It is also not possible to extend the model to environments where there are many projects. There is a threat of one project subsidizing another. The model for each project is tightly coupled that the changes in various stages of the projects needs complex restructuring of the costing.
Explaining Cost Object
The cost can be used as an excellent medium that can organize various types of information of multiple projects. Cost data that usually reside in more than one system are brought together to form a single data model which can effectively generate the real-time information necessary for timely decision making. A cost model in an object-oriented format integrates cost data that otherwise exists in separate information systems.
The use of cost as a means to measure project performance has been widely accepted by the construction industry. Hamburger (1986) presented three perceptions of project costs along the timeline: commitment, expense, and cash outflow. These percepts represent the lifecycle aspect of project cost in various project stages.
To apply the earned value method for time and cost control, the cost information needs to be integrated with time related information (Rasdorf and Abudayyeh 1991). Teicholz (1987) proposed a mapping mechanism that connects cost breakdown structure (CBS) and work breakdown structure (WBS) by allocating costs to work items in proportions. Kim and Ibbs (1992) used an object-oriented method to develop a model that contains cost and time information within the object.
The cost based model discussed in this paper applies the concepts of product model and process model. The development of these models has been attempted by several researchers to model project information. Product models describe the structural composition of products and are useful in organising and communicating the information to the project participants. Process models describe the business activities associated with product lifecycle. In a way the process model represent important steps throughout a project lifecycle.
In contrast with the activity based project models, the cost based project modeling CBPM attempts to model construction projects in terms of cost. CBPM breaks down the project into cost items and hierarchically reorganizes them based on cost oriented relationships.
To model a project using the CBPM, the core element of the cost data must first be defined. This element is referred to as cost object CostObj , which is a data object based on object-oriented
platform. CostObj contains the properties and the behaviours of cost in formation. It is a self-contained and independent object in which the core properties are cost values and timestamps.
As shown in figure 1 CostObj would integrate CBS information, project WBS information, project requirement documents, and project schedule information. It could also contain additional cost information stored within the cost object.
The scheduling of the project activities play an important role and its important to have the scheduling activities revolve around the cost as time has direct impact on it.
The planning and implementation of any project can only be done after it has been analysed and split into manageable tasks. It is necessary for a project to have a fundamental form of Work breakdown structure. Work breakdowns must be carried out in a systematic fashion and structural relationship between the different constituents of work has led to the widespread use of the term 'Work breakdown structure' commonly abbreviated to WBS. (Dennis Lock)
A typical WBS for large project could be assumed to be that of a family tree or that of a jigsaw puzzle. The idea of the whole project could be visualised with all the pieces of jigsaw in right place and no pieces missing. The total costing must be spread over the project work breakdown structure so that there is a specified budget for each work package.
The cost breakdown structure shows details of the cost categories for a project. The CBS is sketched out based on the knowledge of WBS and the statement of work. The CBS is a tool for identifying in one document all the cost categories and arranging them to suit the project purposes. The CBS success depends directly on the quality of the cost-benefit analysis and the feasibility study that was carried out.
The various types of project documents like the Requirements documents (Statements that identify attributes, capabilities, characteristics, or qualities of a system), Architecture/Design documents (Overview of software. Includes relations to an environment and construction principles to be used in design of software components)  are modelled around the cost. This will enable the identification and managements of the costs that are associated with the project documentation.
Integrating product and process model is based on the object-oriented paradigm that defines relationship between design components and project processes. The main features of the object-oriented approach include the data abstraction, encapsulation , inheritance and object relationships.Using CostObj as a core element, project information is modelled in a hierarchical manner. The object-oriented cost information model has three main layers:
1. cost object layer
2. project layer
3. information output layer
Cost Object Layer: The cost object layer (COL) is where the majority of cost data is stored. It contains: a mapping object CCostObjectRoot , a cost object CCostObject. The various sub classes of the project classes can include type specific attributes like requirement cost, testing cost etc. The objects in this layer are the core objects as far as the model is concerned. CCostObjectRoot is equivalent to a cost item and it organizes CCostObject for data processing. The COL is a complete package of cost data, attribute modules, and relationship
definitions. Since the data contained within the COL are plain values of cost and timestamps, the COL has great potential as a self-contained module that can be plugged into the legacy cost oriented information systems.
Project Object Layer: The project object layer POL process the data stored in the cost object layer.As shown in the figure it organises the information into various levels of abstraction by aggregating the cost data residing in the cost object layer. Hierarchical data view is possible in this model. A engineer can have a detailed information about the project costs whereas top management people can take a broader view of the cost data. The cost information processing is predominantly done by the relationship that exists between the CCostobjectroot classes and the Cproject. Any external routines that need to manipulate the cost data according to the users requirements can be implemented using this relationship. Objects that track change in the project is implemented in this layer. The Changeorderroot is the instance of each change order that is processed in the project and it also consists of the Cchangecase. This object is associated with the collection of cost object. CCOCR can have association with the a changeorder class which has the common attributes of the change descriptions, date and time, status, cost estimate. CProject class has an association to CProjectCalendar CPCal class to provide the information output layer IOL a feed of project data.
Information Output Layer: The function of the IOL is to display processed information to the user in a graphical format. There are two objects defined for this operation: information output cell object CInfoOutCell and project calendar object CPCal . A CInfoOutCell stores numeric values in a cell i.e., computer memory cell , then is assigned to a CProjectCalendar which contains a calendar date . The stored cost values in CInfoOutCells can produce a line graph showing the change of the cost over time.
The above figure showsthe steps of data being prepared for information output. A set of raw data is stored in the cost object i.e., CCostObjectRoot . After intensive data processing, the results are placed in the information output object i.e., CInfoOutCell in a much simpler format joined with the project calendar object i.e., CProjectCalendar.The advantage with this is that model does not have to retrieve data every time from the CCostObject layer unless there is a change in the basic raw data, it can get the processed data for display from the CInfoOutcell layer itself.
There are different levels of abstraction that is followed in this model. Even though model provides different levels of information abstraction, it shares the same information regardless of the user levels. At the operational level of the management, the cost information presented must be detailed and accurate. All the cost related interaction in various subclasses of the cost object must be captured. The cash flow profile of a single project can be prepared for project managers whereas similar profiles across multiple projects will be useful for the regional mangers. The collection of such expenditures can then represent the progress of construction activities. The progress of the work packages and ultimately of the entire project can again be derived from the collection of detailed single cost interactions.
The completeness of the information contained is ensured by associating each cost value to a date thus tracking every monetary activity of the project. The Costobjectroot is responsible for organising the data from CCostobject and its subclasses which contains the possible project costs. Each instance of these classes in detail will enable in seeing the entire history of the cost for an item throughout the project. This will also help in the future estimation of similar projects as this data could be archived and later used for indicative purposes.
The integration of the cost data to the project calendar will give the information model a medium to quantify the progress of the project and evaluate the cost performance. Cost performance is usually measured by comparing the earned value to the actual estimated cost of the project. The variance in this value can be used to know the quantum of deviation.This model will help in the extension of the Earned value model to be extended to multiple projects. It allows the project manager to forecast the project's final cost requirements based on its performance against the plan. One of the more beneficial aspects of the earned-value concept retained in this model is its ability to independently forecast the total required funds at the end of a project, commonly called the "estimate at completion." Based on project performance against the plan, a project manager can accurately estimate the total funds required to finish the job within a finite range of values .This will give a fair idea about the success of the project and also constantly able to reschedule the project expectations during its various stages.
Comparison with traditional models
The existing models focus on activities that are organised around the Work Breakdown Structure (WBS).This will limit the organisation wide management of the costs which will require information integration among various projects. By using the object oriented concepts; the cost object integrates the real time cost data across different projects. This will help the management in identification of the underperforming and focus on its shortcomings. The information model used is compact by storing only relevant data in the objects. This allows the easy data retrieval irrespective of the size of the database this model is used with. Data manipulation of this sort was not possible in the earlier models as we had to depend on the activities for getting the cost information.
A project faces many changes and uncertainties during its operation which require necessary measures at the time of the problem's occurrence. The discussed information model focuses on achieving real-time project control. Such real-time project control is only possible when a real-time information system is available. An information system that can analyze the history, check the current status, and forecast the future is essential in the prevention of upcoming problems. The model shows that providing the most updated information to users is possible through the object-oriented design of the information output object CInfoOutCell.
Also it demonstrates the feasibility of integrating data from diverse locations and formats to be synthesized in real-time into useful information for decision making. This concept proves the feasibility to be integrated with existing project management systems to perform project management functions such as earned value analysis for multiple projects on a real-time basis.