Int. J. Production Economics 58 (1999) 289—301 Application of discrete event simulation to the activity based costing of manufacturing systems T.A. Spedding *, G.Q. Sun Centre for Engineering and Technology Management, School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798, Singapore CIMTEK Pte Ltd, 151 Chin Swee Road, C16-02 Manhattan House, Singapore 169876, Singapore Received 28 April 1997; accepted 6 August 1998 Abstract In the last two decades traditional cost accounting practices have been unable to respond to the changing information needs of manufacturing management. Activity Based Costing (ABC) is a method which can solve many of the limitations of traditional cost systems. This method of accounting involves the breaking down of the individual activities and costing of the amount of time spent on each step of the manufacture of a product. This paper illustrates how Discrete Event Simulation may be used to evaluate the ABC of a manufacturing system. A visual interactive simulation software WITNESS is used to model a semi-automated Printed Circuit Board (PCB) assembly line. The PCB assembly line case study demonstrates how ABC can be applied to a manufacturing system using simulation modelling techniques. The paper also discusses further applications of ABC in the manufacturing environment and includes a case study on the operational quality cost. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Activity Based Costing; Discrete Event Simulation; Manufacturing systems 1. Introduction In the last two decades, competition in the global manufacturing environment has greatly intensified. For companies to survive and maintain profit margins, cost management techniques must constantly be improved [1]. Traditional cost accounting practices have been unable to respond to the changing information needs of manufacturing management. * Corresponding author. Tel.: 65 7995589; fax: 65 7911859; e-mail: mtspedding@ntu.edu.sg. Today managers are finding that traditional methods are unable to support decision making in costing. Many companies are finding that traditional accounting methods are “too late, too aggregated and too distorted” to support decision making in costing [2]. Activity Based Costing (ABC) can help rectify the shortcomings of the traditional cost systems. Although researchers like Cooper, Kaplan and Turney have successfully explored cost theories and the development of ABC (see for example the papers of Cooper and Kaplan [3—5]), they have not directly addressed the issue of effective transfer of 0925-5273/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved PII: S 0 9 2 5 - 5 2 7 3 ( 9 8 ) 0 0 2 0 4 - 7 290 T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 cost technology to the manufacturing environment [6]. Without an appropriate computer model, the number of activity combinations and cost item variations required in ABC are extremely time consuming and costly, making implementation of the ABC difficult [7]. Ideally, a dynamic modelling framework that can include both the operational aspect of a manufacturing system and financial variables needs to be developed for ABC. Only then can management move beyond the traditional static investment analyses of manufacturing systems. Simulation models are developed by observing the actual processing times of activities in a system and then characterising their variation by statistical distributions. When the simulation model is run a time advance algorithm is used for event scheduling. Random numbers from the observed statistical distribution are then generated to represent the duration of activities. The activity based cost associated with the activity can then be generated by multiplying with the time. Accurate and detailed ABC models are necessary for process improvement and process redesign projects (see [8, Ch. 8]). Such projects are usually focused on processes and activities, as are traditional simulation models. Incorporating ABC into such projects provides an extra strategic dimension to the model in terms of costing [9,10]. The accuracy usually depends on the detail of the ABC model and the type of cost driver used. Developers have a choice of three types of activity cost drivers which are, in order of increasing accuracy (and cost of measurement): transaction drivers which count each time an activity takes place; duration drivers which represent the time taken for each activity and so takes into account variation; and intensity drivers which directly cost for the resources used each time an activity takes place. Simulation based ABC has the ability to use any (of a combination) of the three drivers. However, simulation models naturally lend themselves to using duration drivers based on the random processing time of activities. More complex models can be developed to incorporate intensity drivers where necessary. ABC systems relate organisation spending of recourses to the activities and business processes performed by these resources. At the facility level Activity cost drivers are used to collect information that is then driven down to the products, services and customers that create the demand for the facilities activities [8]. Activities can be classified into four general categories [11]: 1. ºnit level performed each time a unit is produced. 2. Batch level performed each time a batch of goods is produced. 3. Product level performed as needed to support the production of each type of product. 4. Facility level performed to sustain the factory’s performance such as rent, depreciation, insurance etc. ABC has the conceptual advantage of providing the option of viewing ABC’s facility costs as period costs or allocating them at the batch or product level. An example of the use of this classification can be found in [12]. In ABC, variable overhead costs are traced to individual products, however it is not always straightforward to assign fixed overhead costs to unit drivers. ABC systems have the advantage of associating many of the costs that are defined in traditional costing systems as fixed overheads with changes other than production volume. ABC therefore helps to clarify the relationship between the causes of cost increase and decrease and the individual products. It is this cause and effect relationship which allows management to differentiate between value added and nonvalue added activities ABC systems therefore have the potential to be a Strategic Decision making tool for process redesign and continuous improvement [13]. As the simulation model tracks the number of products at each stage of the manufacturing process, batch level and product level costs can be T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 added at the appropriate stages. Facility level costs can be allocated to individual products or treated as period costs (see for example [11]). Discrete event simulation software provides a valuable tool for the development of ABC in a manufacturing environment. First, the execution of a discrete event simulation model is based on an event or activity that occurs in a manufacturing system. The ABC model can be easily developed by adding the cost attribute into existing simulation reports. Secondly, recent developments of the animation and data interface of simulation software allow ABC models to be graphically presented and integrated into the company’s information system. Thirdly, simulation has been widely applied in manufacturing companies for the improvement of productivity. ABC is therefore a natural extension of current simulation applications. This paper illustrates the application of simulation to develop ABC models for use in the manufacturing industry. A model of a semiautomated Printed Circuit Board (PCB) is developed in WITNESS; a visual interactive simulation software. The PCB assembly line case study demonstrates how simulation modelling can be used for a manufacturing system. Several examples are also presented which demonstrate the flexibility and potential of simulation-based ABC models. 291 2. Activity based costing ABC was introduced by Kaplan and Cooper of Harvard Business School as an alternative to traditional accounting techniques in the 1980s (the earliest papers include Cooper and Kaplan [3—5]). Many have since used this method for product costing in both manufacturing and business applications [14]. The ABC method of accounting involves the breakdown of a system into individual activities and costing of the amount of time and resources spent on each activity in the manufacture of a product. A schematic diagram to illustrate this point is given in Fig. 1. Traditional accounting methods concentrate on volume-based costs systems and these methods are highly inaccurate in the modern manufacturing environment. Today much of the significant cost in producing an item are not volume related, for example the cost of engineering, order processing, planning, quality control, etc. for high technology, make-to-order products, or just-in-time delivery. ABC, however, takes into account the cost incurred at the activity level and then attributes the cost to products according to the activities that a product goes through. Even with a simple case ABC can be very complex and time consuming, that is why it is not Fig. 1. The activity based approach is based on an understanding of how activities use resources. 292 T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 widely applied in the manufacturing industry. It is recognised that ABC can be used to enhance rather than replace the accounting system when the company finds it too difficult to implement full-scale ABC-based accounting. In this case, ABC is used as a cost management tool by the operational departments to compliment the existing accounting practice. To the operational staff, implementing ABC using simulation models can be far more convenient, spontaneous, straightforward, and interactive than on an accounting software. Simulation models can provide the flexibility required by ABC to cope with the changes in combinations and permutations often encountered in a manufacturing system. Although adopting an ABC accounting system has its benefits over traditional methods it cannot solve all the problems related to product costing. Accuracy will be dependent on other factors such as the activities, cost pools and cost drivers chosen by the system users. A large difference can be caused by cost drivers that have been left out of the ABC model. In ABC cost pools must be identified for each activity. The cost is incurred once the product passes through the activity. The factor that causes the cost to be incurred in the activity is known as the cost driver. The development of an ABC model is relatively simple. A straightforward procedure is discussed in Lewis [15]. 3. Implementing ABC in simulation models To include ABC into the simulation model, calculation of the cost of each activity must be included in the simulation model. Eq. (1) below shows how this is done: C"¹R#M (1) where C is the cost contributing to the making of the product, ¹ the time during which the resources are occupied, a parameter that must have been built into the simulation model, R the cost rates at which the resources are being used (this may include labour, depreciation and utilities rates as in accounting, as well as other significant cost such as engineering, planning, quality control, etc. depending on the cost drivers specified) and M the cost of material used for the process (also included in traditional accounting methods). When the total cost of making a product is calculated, the cost of all the activities need to be added together. Therefore, the cost of making the ith product in a manufacturing system can be given as HL C" C G GH H (2) where C is the cost required to make the ith prodG uct, C the cost of performing the jth operation GH obtained from Eq. (1) and n the total number of operations needed to produce the ith product Because the operations through which a product is processed have been built into the simulation model, the result of Eq. (2) is automatically reported by simulation if Eq. (1) is incorporated into the model. The only variable that needs to be decided in order to use simulation for ABC, as seen from Eqs. (1) and (2), is the cost rate. The cost rate is determined when the cost pools are identified and the cost drivers of each activity detailed. 3.1. Application of ABC to manufacturing using simulation To demonstrate how ABC can be used in a manufacturing environment a simple model of a semi-automated printed circuit board (PCB) assembly line was created in WITNESS and costed using ABC techniques. WITNESS, a menu based visually interactive simulator software, is designed to build complex continuous or discrete event models [16]. The modelling is simplified by using graphically interactive forms to build the models. The accuracy of the models may be controlled with a large number of predefined and user defined functions. The way parts are pulled or pushed in and out of the elements within the model are controlled by the input/output rules. Reports and summary information are useful tools for error checking during model building. These and other features in T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 WITNESS can facilitate the building of a model to calculate ABC. The basic model consists of four machines, a solder paste machine, a device placement machine a reflow soldering machine and an inspection machine. The first three machines are linked by conveyors. Fig. 2 shows the flow process. The model developed in WITNESS is shown in Fig. 3. The top window shows the process flow, where PCB are processed through solder print, device placement, reflow, and then inspection. Machines are connected by conveyors. The lower window dynamically displays the activity based cost. Distributions of process times of the machines and conveyors and costings were taken from an actual PCB Assembly line. The results were verified using data from a previous study of ABC analysis applied to a PCB assembly line which was based on 293 an IDEF modelling approach [17]. When run under the same conditions the simulation model gives the same results as the IDEF model. However, the simulation modelling approach to ABC is more flexible and detailed and can take into account the intrinsic variation of the manufacturing process. Fig. 4 shows the graphical results from the WITNESS model. The percentage values are calculated and displayed as pie charts by the software. The results show that savings in operation cost can be made by reducing setup times of the placement and reflow machines. Further savings can be made if operators with lower wages are used for the operation of solder and reflow machines. 4. Applications of ABC in manufacturing The previous section has illustrated how a simulation for ABC can be developed for a manufacturing system. This model can be used to provide accurate on-line monitoring to facilitate the costing of the day to day running of a manufacturing. A particular advantage of using a simulation models that it can be fast-forwarded into the future to obtain realistic projections of further running costs and expenditure. Also a dynamic environment is naturally accounted for in the modelling procedure. Once a simulation model for the ABC of a manufacturing system has been developed it may be extended to analyse other aspects of the manufacturing strategy. This section presents some examples which demonstrate how the basic simulation model for ABC can be extended to provide more specific detailed analyses of a manufacturing system. 4.1. Calculation of surplus capacity In the traditional costing method, the “overhead” cost (total cost-direct labour cost) is calculated from the historical record, and then proportionally added into that of new products. It greatly distorts the true cost that may be needed for making the new products because: Fig. 2. Flow process of the model. 1. the overhead may not be proportional to the direct labour time, particularly in a manufacturing 294 T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 Fig. 3. WITNESS model of PCB assembly line. Fig. 4. Graphical output of WITNESS model. T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 295 environment with high development and engineering contents; 2. the concept of lump sum “overhead” misleads the operations to believe that the historical performance is normal, thus hiding the improvement opportunities. based cost. Surplus capacity depends on the utilisation rate of the resources and therefore similar analysis can provide a useful indication of where to improve resource allocation along the assembly line. ABC however, only takes into account the cost of the activities that contribute to making new products. It considers the cost of non-value-added activities such as machine/labour idle time as surplus capacity (or redundant investment for companies with insufficient work), which can be used for taking more business. The cost of surplus capacity can be obtained from the following equation: 4.2. Improved cost allocation GK S"¹ ! C , (3) G G where S is the surplus capacity, ¹ the total cost of running the manufacturing system, C the the cost G required to make the ith product and m the total number of products produced. Fig. 5 shows simulation of the SMT line over a 24 h period. Surplus capacity is shown as the difference between the total cost and the activity In traditional costing techniques indirect cost is allocated on the basis of available machine hours and is therefore independent of the resources required for a particular operation. Fig. 6 shows a comparison between traditional techniques and ABC when applied to the product costing of three products P1, P2 and P3. The left side of the chart is the costing by traditional accounting methods, where manufacturing cost is proportional to the processing times of products because all go through processes with the same cost rates. P1 is shown to be the least and P3 the most profitable product. On the right-hand side, however, ABC allocates the significant engineering cost by the actual setup incurred on the products, thus resulting a more realistically low manufacturing cost for P1 and high one for P3. In this case the differences between the Fig. 5. Dynamic display of surplus capacity. 296 T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 Fig. 7. “What—if ” analysis showing improved cycle times. Fig. 6. Improved cost allocation. profitability of different products should be marginal. This example illustrates one of the fundamental limitations of traditional costing procedures in aggregating costs leading to inaccuracies in cost allocation. ABC, however, distinguishes costs at the activity level resulting in improved accuracy. 4.3. Justifying capital investment An advantage of simulation-based ABC is that all the traditional “what—if ” techniques usually associated with simulation models can be carried out. Fig. 7, for example, represents a comparison of the cost of making 500 PCB’s when the cycle times of the 4 machines are slightly altered (all other factors are unchanged). This could represent a situation where a new PCB is being assembled or different machines or methods are employed. The results show an increase of 2.2% in the cost of production, illustrating the potential of ABC in providing a reliable decision support mechanism by reflecting relative minor changes in operational strategy. A second “what—if ” scenario demonstrates the potential of introducing a second machine to relieve a bottleneck. A bottleneck is introduced into the original model by increasing the cycle time of the placement machine. Once the results of this scenario are obtained, a second placement machine is introduced into the model. A comparison of the results are shown in Fig. 8. A 69.4% increase in production output is obtained at a cost increase of 0.8%. Further analysis can determine the capital Fig. 8. “What—if ” analysis showing the addition of a second placement machine. investment by taking into account the net cash flow over an extended period using the net present value technique [18]. The advantage of using simulation based ABC analysis is that hidden costs are shown such as maintenance and the consumption of utilities. With the detailed information obtained from ABC it is possible to determine if overall cost savings or increased revenues can be made with additional capital investment. 4.4. Quality costing Simulation based ABC is a powerful tool for the measurement and analysis of quality costs. An important strategic decision for example is where to position inspection stations on a manufacturing line. An equitable balance needs to be obtained between improved quality and less rework against increased inspection costs and slower throughput. Fig. 9 illustrates the output of the simulation model when an extra inspection stage has been added between the first and second machines. If a defective PCB is made at the beginning of the T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 297 Fig. 9. Addition of extra inspection stage. Fig. 10. Rework cost against number of reworks. manufacturing process and left undetected until the final inspection stage, the cost incurred is obviously higher than if it is discovered earlier. Break-even analysis can then be performed to help determine the number and position of inspection stages. Fig. 10 illustrates the difference in the cost of defects when detected early compared with those left until the final inspection stage. Traditional quality costing techniques adopt the Prevention Appraisal Failure (PAF) Model where quality costs are divided into the three categories of prevention, appraisal and failure [19]. This model can easily be applied using simulation-based ABC and has the advantage of being universally accepted (BSI 1990) and so is very useful for benchmarking activities, etc. However the model has several notable limitations [20]. The British Standards have included an alternative approach to quality costing based on process modelling [21,22]. The technique focuses on departmental objectives and process ownership and identifies all the parameters and activities within the process to be monitored and subsequent costs of conformance and nonconformance. The process modelling technique is therefore naturally suited to simulation based ABC models. In a recent study Ittner [23] indicated the need for greater use of dynamic models for quality management and the need to abandon aggregate costing techniques that fail to distinguish between prevention and appraisal costs. Simulation based ABC has the advantage of providing both these characteristics. 5. Case study The example of PCB manufacturing showed that ABC is ideal to quantify the significant cost of engineering, which is not volume based. For 298 T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 manufacturers of make-to-order products, the cost of marketing, planning, quality control, etc. takes up a large portion of the operating cost. ABC results in a far more accurate costing picture than the traditional accounting method. To illustrate the potential of simulation based activity based costing a case study was conducted in collaboration with a local (Singapore) manufacturing company. The object of the project was to benchmark the operational quality costs incurred by the company with the industrial norm in order to determine the efficiency of the quality control. The logical and physical process flow (Fig. 11) for a single product type was modelled over a period of one year. This model was verified and validated in collaboration with the company before details of the costs for each process were added to the model. The simulation of the manufacturing system thus assumed the form of a natural process cost model (see Section 4.3 and [21,22]) which gives a more accurate cost breakdown and a better means of quantifying quality costs. The process in this case is synonymous with the activity and so costing can be realised as a natural consequence of the discrete event simulation model. The quality costs produced by the simulation model were compared directly with the industry norm using figures published by the National Productivity Board of Singapore [24] (Fig. 12). Results Fig. 11. Schematic process flow diagram. Fig. 12. Quality costs for manufacturing. T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 of the simulation model either concurred directly or were very close to the theoretical observations. Scenario analysis was then performed to investigate how quality costs could be further reduced. The results showed that significant improvements could be made by improved utilisation of labour. In the existing practice, the cost of many activities were proportionally distributed to products by the end of an accounting period. The ABC study, however, revealed that the difference between cost incurred to different products are very high, e.g. operations quality planning (OQP), inspection of purchased supplies, etc. The simulation exercise provided the company with a better insight into their quality costs as well as a powerful decision support for further improving the mechanisms underpinning their quality performance. 6. Discussion The previous two sections have illustrated several applications of simulation-based ABC including: E E E E E E Simultaneous Costing of Manufacturing System Calculation of Surplus Capacity Justifying Capital Investment Improved Cost Allocation Quality Costing Evaluation of New Technology Other applications include: E “What—If ” scenario for scheduling/manufacturing strategy, etc. E Product mix decisions by selecting the most profitable product E Extension of the model to include the business side of the manufacturing enterprise — to help measure and justify Business Process-Re-engineering (BPR) Once a simulation model has been developed which accurately reflects the costing of a manufacturing system, probably its most significant application is for the on-line monitoring of a system so that day-to-day expenditure can be accurately determined and potential sources of cost problems can be identified. However once the model has been 299 developed and validated its potential for analysis and decision making is considerable. The previous section has illustrated some of the applications and the graphical results demonstrate the usefulness and hence the advantage of simulation-based ABC as a communications and decision support tool. Other advantages of simulation based ABC include: E ABC software should be accessible [25] and simulators and simulation languages are relatively easy to learn and use. Most contemporary simulators and simulation languages are well established, have a user friendly environment, require little or no programming experience, and provide excellent documentation and on line support. E Several recent editions of simulation software have a costing module which can be adapted to perform ABC with minimal programming. E Simulation environments have error checking features which ensure the validity of both the operational and financial aspects of the model. The flexibility of the software facilitates the extension of ABC from a basic model to a large variety of applications such as those illustrated in Section 4. E Graphical routines such as those supplied by WITNESS (Figs. 4 and 5) and other simulation software provide an automatic and powerful visualisation tool for the analysis of simulation output to aid the decision-making process. E Graphical animation produced by most simulation software provides a dynamic time-based visualisation of potential problems resulting from exorbitant costs. E Simulation is time based and because it models the stochastic nature inherent in a manufacturing system the intrinsic variation and dynamic behaviour is characterised which could be highly significant when estimating costs. However, there are several limitations when implementing simulation based ABC models. These include: E Cost allocation in any ABC model (regardless of whether or not it is based on simulation) can cause difficulties as certain costs cannot be 300 T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 allocated to a particular activity. Some administration, business, or preparation work, for example, cannot be associated with a particular activity — one solution may be to combine the use of ABC with a more traditional costing method. E When cost information is added into the model the effort required to determine the cost driver and the simulation time is considerably increased. A detailed model, in particular, may require significant computational time to obtain results. A decision needs to be made by any company to introduce ABC: at which level will the activities be used, e.g. process level (or business department), work centre level (or business step) or machine level (or staff ). E The addition of cost information also adds an extra dimension of error. Verification and validation of any simulation model is difficult. It is usually more difficult to verify the cost information provided by simulation models as the original information is not available in such detail. Simulation-based ABC has several notable advantages. Choice of simulation software, however, requires careful consideration. One of the overriding consideration in simulation is the building of the model. Although this process has several notable pitfalls the procedure is well documented (for example, [26]) and because simulation modelling is a mature and proven methodology it is probably more reliable than many other techniques. Verification and validation are other critical issues and as previous cost estimates tend to be aggregated it is often difficult to provide detailed analysis for comparison. However, the advantages of an accurate modelling technique for ABC which provides an on-line costing mechanism plus a versatile and flexible decision support tool far out weigh the limitations. 7. Conclusions Accurate costing information has become one of the principle contributions to the decision making process in the manufacturing environment. Simulation results involving the increased throughput or production time are not enough for management to make decisions. Furthermore, outdated accounting methods are unable to respond to their information needs. Activity Based Costing can help evaluate costs more effectively than traditional accounting methods. This can provide an important advantage to companies in both the service and manufacturing environment. Without the flexibility of a computer simulation model, the number of combinations and testing variations required by ABC would be extremely time consuming and costly, making implementation difficult. The example of a PCB assembly line shows how ABC can be implemented in the simulator WITNESS. The model is able to generate accurate results of an ABC analysis. When run under similar conditions the simulation model gave the same estimates of cost as those derived from an IDEF modelling approach. However simulation models have the advantage of being able to provide greater detail and take into account the intrinsic variation of a dynamic manufacturing system. The use of discrete event simulation for ABC can easily be extended to the analysis of specific applications within the manufacturing environment. These include efficient cost allocation, calculation of surplus capacity, management of quality costs and capital justification. Computer based simulation of the ABC of a manufacturing system provides a powerful tool for management decision making. Process improvement and reengineering strategies can also be investigated using similar techniques. Here the traditional ABC model is extended to encompass Activity Based Management (ABM) [27] which applies activity based analysis to process cost reduction. Acknowledgements The authors would like to express their gratitude to Mr Clayton Lee and Mr Andrew Lee for assistance in the development and implementation of the simulation models presented in this paper. References [1] M. Morrow, Activity Based Management, WoodheadFaulkner, New York, 1993. T.A. Spedding, G.Q. Sun/Int. J. Production Economics 58 (1999) 289—301 [2] T. Johnson, R.S. Kaplan, Relevance Lost, Harvard Business School Press, Boston, MA, 1987. [3] R. Cooper, R.S. Kaplan, Measure costs right: Make the right decisions, Harvard Business Review (1998) 97—98. [4] R. Cooper, R.S. Kaplan, How cost accounting distorts product cost, Management Accounting 22 (1998). [5] R. Cooper, The rise of activity based costing: Parts 1, 2, 3 & 4, Journal of Cost Management (1988—1990) [6] J.S. Zuk, G.B. Kleindorfer, W.B. Nordgren, R.D. 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