تجزیه و تحلیل مقایسه ای استفاده از هزینه یابی بر مبنای فعالیت و تئوری محدودیت ها برای تصمیم گیری های آمیخته محصول

Activity-based costing (ABC) and the theory of constraints (TOC) represent alternative paradigms for evaluating the economic consequences of production-related decisions. However, their application can lead to contradictory product-mix decisions. To resolve this conflict, it is frequently suggested that the TOC is appropriate for the short run, while ABC is appropriate for the longer term. This paper models the selection of a product mix with the TOC and an ABC model integrating activity-based cost with the capacity of production-related activities. The paper demonstrates that management's discretionary power over labor and overhead resources determines when the TOC and ABC lead to optimal product-mix decision. Equally important, it demonstrates that both the TOC and ABC may lead to a suboptimal product mix across a wide range of economic conditions. The paper develops a more general model of the product-mix decision and demonstrates that the TOC and ABC are special cases of this model. Finally, the paper discusses how the general model may be used to supplement information provided by the TOC and ABC.

Activity-based costing (ABC) and the theory of constraints (TOC) represent alternative paradigms to traditional cost-based accounting systems. Both paradigms are designed to overcome limitations of traditional cost-based systems and, thereby, provide more relevant information for evaluating the economic consequences of resource-allocation decisions. While their objectives are similar, the means used to achieve these objectives differ significantly. ABC models the causal relationship between products and the resources used in their production. This enables ABC to provide more accurate product-cost information for evaluating the profitability of the firm's product lines and customer base [1]. Conversely, the TOC represents an application of general systems theory for optimizing production. It uses the most constrained of the firm's activities to guide production and process improvement decisions. Firms adopting the TOC indicate that it has aided in reducing lead time, cycle time, and inventory, while improving productivity and quality [2]. One of the questions confronting many managers today is deciding which paradigm to select for production-related decisions. Studies comparing the decision usefulness of ABC and the TOC are contradictory. Low [3] and Spoede et al. [4], using numerical examples, illustrate that the TOC leads to a more profitable product mix than ABC. Low ([3], p. 36) noted that the “activity-based cost allocation procedure was a great deal more complex than traditional costing procedures, but it was not particularly helpful in a strategic sense”. Kee [5], using a similar example, illustrates that an ABC model integrating the cost and capacity of production activities outperforms the TOC. The generalizability of these studies is limited due to their use of numerical examples to illustrate the relationship between the TOC and ABC. Consequently, more rigorous analysis is needed to assess the generalizability of the Low [3], Spoede et al. [4], and Kee [5] studies and to reconcile their contradictory conclusions. The complementary nature of the TOC and ABC has been examined by Bakke and Hellberg [6], MacArthur [7], and Holmen [8]. They suggest that the TOC is appropriate for the short run, while ABC is appropriate for longer-term decisions. However, as noted by Bakke and Hellberg ([6], p. 13), there is no clear-cut demarcation between short-term and long-term decisions and short-term decisions may have longer-term economic consequences. Time is a surrogate in these studies for other factors in the firm's operations that determine when the TOC and ABC lead to optimal resource-allocation decisions. However, the nature and impact of these factors on ABC and the TOC were not addressed. Accordingly, determination of these factors is crucial for understanding the strengths and limitations of the TOC and ABC as information systems. This study examines the economic conditions under which the TOC and an ABC model incorporating the cost and capacity of production activities lead to an optimal product-mix decision. It demonstrates that the TOC and ABC lead to an optimal product mix under specific sets of economic conditions. Equally important, it also illustrates that both models may lead to a suboptimal product mix across a wide range of economic activity and suggests an alternative model that may be used to supplement information provided by the TOC and ABC individually.

The theory of constraints (TOC) and activity-based costing (ABC) assume that a firm's management has either no control or has complete control over its labor and overhead resources, respectively. When the respective assumptions are met, the TOC and ABC lead to optimal product mix decisions. However, when a firm has varying degrees of control over labor and overhead resources, neither the TOC nor ABC may lead to an optimal product mix. A more general model was developed in the paper that overcomes the stringent requirements of the TOC and ABC. The general model subsumes the TOC and ABC as special cases. Therefore, it may be used to supplement information from these paradigms. For example, the general model may be used to modify the TOC to reflect the economic attributes of a firm's production resources, rather than forcing these resources to fit the assumptions of the TOC. Equally important, the more general model may be used to identify a bottleneck and the unused resources in the firm's other production activities. This information may be used to identify where protective capacity may be needed to mitigate the effects of the stochastic properties of the firm's production processes. Identification of a bottleneck and the excess capacity of other production activities may be used to stimulate efforts to relieve successive bottlenecks. In effect, the general model may be used to implement the TOC's process of continuous improvement. The product mix that maximizes the profitability of the general model occurs when a firm's management has control over labor and overhead resources, which is equivalent to the product mix identified with ABC. In effect, ABC identifies the product mix that is the most useful for the firm to produce from a strategic perspective. However, the profitability identified with ABC requires the firm to have discretionary power over its labor and overhead resources. For firms selecting a product mix with ABC based on strategic considerations, the difference in the profit of the product mix selected with ABC and the general model measures the opportunity cost of using ABC when its assumptions with respect to labor and overhead are violated. The non-discretionary resources identified from the general model represent resources the firm must gain control over to attain the profitability of the product mix identified with ABC. As the firm is able to convert non-discretionary to discretionary resources, the product mix identified with the general model approaches that of ABC. Comparative analysis of the TOC and ABC has been restricted to the product-mix decision. However, the TOC and ABC are used across a much wider range of economic activity. Consequently, comparative analysis of the TOC and ABC for implementing a process of continuous improvement, redesigning and pricing products, outsourcing, and acquiring capital assets needs to be examined. Also, the TOC and ABC provide information for stimulating organizational learning and change; however, scant information about these implications of the TOC and ABC has been examined. Consequently, surveys and field studies of firms that have used the TOC and ABC are needed to understand how each paradigm performs in practice. An analysis of operational and financial attributes of firms that have used the TOC and ABC may be used to evaluate their relative strengths and limitations with respect to implementing a process of continuous improvement, redesigning and pricing products, outsourcing, and acquiring capital assets. Equally important, this analysis may be used to evaluate the cost and benefit of using each paradigm and its relative superiority for making different types of production-related decisions.