Difficulty: Intermediate. Prerequisites: cost behaviour and CVP analysis (Doc 1).
Traditional costing systems lump overhead into one or two pools and allocate using a volume-based driver such as direct labour hours, which distorts product costs when products consume resources differently. Activity-based costing (ABC) fixes this by creating many homogeneous cost pools, each with a driver that reflects actual resource consumption. Time-driven ABC (TDABC) simplifies implementation by computing a cost-per-minute for each resource and multiplying by the time each product or service actually uses.
Cost pool
A grouping of individual indirect cost items. Overhead is collected into cost pools before being allocated to products. In simple terms, a bucket that holds related overhead costs.
Cost driver (allocation base)
The factor used to assign costs from a cost pool to cost objects. A good cost driver is causally related to the costs in its pool. Examples: machine hours, number of setups, number of inspections.
Overhead rate
The cost allocated per unit of the cost driver, calculated as total overhead in the pool divided by total volume of the driver.
Traditional two-stage cost system
Stage one allocates overhead to departments or cost pools. Stage two allocates department overhead to products using a single volume-based driver (typically direct labour hours or machine hours).
Activity-based costing (ABC)
A costing method that assigns overhead to activities (not departments), identifies a cost driver for each activity, and allocates costs to products based on each product's actual consumption of each activity.
Time-driven activity-based costing (TDABC)
A simplified version of ABC. For each resource group, compute a cost per minute (annual cost of the resource / practical capacity in minutes). Then estimate the time each product, service, or customer requires from that resource and multiply.
Overcosting / undercosting
A product is overcosted when the cost system assigns it more overhead than it actually consumes, and undercosted when it assigns less. Overcosted products appear less profitable than they are; undercosted products appear more profitable.
Product diversity
The condition where products differ significantly in their consumption of overhead resources. High product diversity makes single-pool allocation unreliable.
Death spiral
A feedback loop where a company allocates fixed costs over actual (declining) volume, inflating per-unit costs, leading to price increases, further volume loss, and still higher per-unit costs. The fix is to allocate fixed costs over practical capacity, not actual volume.
Theoretical capacity
The maximum output possible with no downtime at all, no maintenance, no breaks. An ideal but unrealistic benchmark.
Practical capacity
The maximum output achievable after deducting expected downtime for scheduled maintenance, breaks, and holidays. The preferred denominator for overhead rates because it does not fluctuate with demand.
Joint products
Two or more products that result from a single production process with a common input. The costs incurred before the split-off point are joint costs.
Split-off point
The stage of processing where joint products are separated and become individually identifiable.
Byproduct
A joint product with relatively low sales value compared to the main product.
Ratio of cost-to-charges (RCC)
A traditional hospital costing method that assumes costs are proportional to charges. TDABC replaces this by measuring the actual time and resources each procedure consumes.
Direct costs (materials, labour) are traced to products. Overhead is collected into cost pools, then allocated to products using a cost driver. The traditional system typically uses one or two cost pools with a volume-based driver (direct labour dollars, machine hours).
Product cost under a traditional two-pool system: direct material cost + direct labour cost + (overhead rate for pool 1 x driver units) + (overhead rate for pool 2 x driver units).
The fundamental problem: when products differ in how they consume overhead, a single volume-based driver misprices them. High-volume, simple products get allocated too much overhead (overcosted). Low-volume, complex products get allocated too little (undercosted).
ETO tests electronic components. Its existing system uses one cost pool allocated by direct labour dollars. The overhead rate is $4,713k / $3,260k = 145% of direct labour.
The problem: testing is becoming more capital-intensive and less labour-intensive. Products differ in whether they use expensive automated equipment or simple labour-intensive rigs. A single labour-based driver treats them all the same.
With one cost pool: products that use expensive machines are undercosted (they consume more overhead per test than the rate reflects). Products with long labour hours are overcosted.
With two cost pools (admin/technical at $685k allocated by DL, and test room at $4,029k allocated by machine hours at $80/hr): costs shift significantly. The ICB, which uses 40 machine hours, now shows a cost of $5,661 rather than $5,025.
With three cost pools (admin, main test room at $2,103k / 33k machine hours = $63/hr, mechanical test room at $1,926k / 17k machine hours = $113/hr): further refinement. Products that use the expensive mechanical room now bear a higher rate.
The three-pool system is intuitively the most accurate because it reflects the fact that overhead is consumed differently in different parts of the production process. The trade-off: more pools mean better accuracy but higher cost of maintaining the system.
When a new, expensive machine is added, it should be treated as its own cost pool to avoid distorting costs of products that do not use it.
ABC addresses the limitations of traditional systems by creating many cost pools, each tied to a specific activity, with a driver that reflects what causes costs in that activity.
Four steps to develop an ABC system:
Identify the activities (using the cost hierarchy: unit-level, batch-level, product-level, facility-level)
Identify the cost driver for each activity (a factor causally related to the cost)
Compute a cost rate for each driver (estimated indirect cost in the pool / estimated volume of the driver)
Assign costs to products (cost driver rate x number of driver units consumed per unit of product)
Traditional systems vs ABC:
Traditional: few cost pools, heterogeneous pools, volume-based drivers, two stages (overhead to departments, departments to products)
ABC: many cost pools, homogeneous pools, drivers matched to activities (not necessarily volume-based), two stages (overhead to activities, activities to products)
ABC is most valuable when indirect costs are large relative to direct costs, when products are diverse in their resource consumption, and when the existing system shows "red flags" (products that should be profitable are not, or customers keep buying supposedly unprofitable items).
Wilkerson makes valves, pumps, and flow controllers. Under the old system, all manufacturing overhead ($806k) is pooled and allocated at 300% of direct labour dollars.
Valves: high-volume, standard, only 10 production runs and shipments. Under ABC their cost falls because they consume little batch-level and product-support activity.
Flow controllers: low-volume, highly customised, many different types. Under ABC their cost rises because they consume disproportionate setup, production-control, and engineering support.
Pumps face price pressure in a mature market. Flow controllers could bear a price increase without losing demand.
The ABC system reveals the cross-subsidy: valves were overcosted and flow controllers were undercosted. Using practical capacity rather than actual volume as the denominator prevents the death spiral (where declining volume inflates per-unit fixed costs, pushing prices up and volume further down).
Anagene is a biotech start-up producing DNA analysis cartridges. Its margins fluctuate confusingly because the overhead rate is recalculated each period based on actual volume.
When volume drops (reduced demand, seasonal dip, start-up excess capacity), the overhead rate jumps: same fixed overhead divided by fewer units. Products appear more expensive, triggering price increases that further suppress demand.
The fix: use practical capacity as the allocation base. This keeps the per-unit rate stable. Unused capacity cost is reported separately as a period expense, making excess capacity visible to management rather than buried in product costs.
Theoretical capacity: production under ideal conditions with zero downtime
Practical capacity: production accounting for scheduled maintenance, normal breaks, and holidays
Anagene uses practical capacity to stabilise its overhead rate
BCH found that its traditional ratio-of-cost-to-charges (RCC) system produced uniform costs across very different procedures. Three procedures all cost $210 under RCC, but their actual resource consumption differed enormously.
TDABC approach:
Calculate the cost per minute for each resource type (surgeon: $522,720 / 87,120 minutes = $6.00/min; ASR: $1.00/min; registered nurse: $1.50/min; clinical assistant: $0.80/min)
Map how many minutes of each resource each procedure actually uses
Multiply and sum
Results: plagiocephaly costs $154.50 (TDABC profit $69.50 at a $224 reimbursement), neoplasm skin excision costs $221.50 (TDABC profit $2.50), craniosynostosis costs $293.00 (TDABC loss of $69.00). Under RCC, all three showed a uniform $14.00 profit.
The lesson: systems like TDABC are needed to attribute costs accurately when different products, services, or customers place very different demands on resources. Simple systems distort decision-making in complex environments.
Wendy's uses overcooked (burnt) hamburger meat to make chili. Ground beef is a joint cost. The hamburger and the chili are closely related products.
The question: should the cost of beef be included in the chili's cost? The answer depends on the decision:
If evaluating chili's standalone profitability: the burnt meat has no alternative use (its opportunity cost is near zero), so assigning the full cost of beef to chili overstates its cost. The cost of beef should not be assigned to chili.
But evaluating chili's standalone profitability arguably makes no sense, because hamburgers and chili are part of the same business model. Chili is a quick-service, high-quality menu item that supports the brand. Costs and profitability should be assessed together.
Key joint-cost concepts:
Joint products share a common input up to the split-off point
Joint costs cannot be meaningfully allocated to individual products before that point
Main product: the product with the highest sales value from the joint process
Byproducts: products with relatively low sales value (example: poultry feathers from chicken processing)
\text{Overhead Rate} = \frac{\text{Total Overhead in Pool}}{\text{Total Volume of Cost Driver}}\text{Product Cost} = \text{Direct Materials} + \text{Direct Labour} + \sum (\text{OH Rate}_i \times \text{Driver Units}_i)\text{TDABC Cost per Minute} = \frac{\text{Annual Cost of Resource}}{\text{Practical Capacity in Minutes}}\text{TDABC Product Cost} = \sum (\text{Cost per Minute}_j \times \text{Minutes of Resource } j \text{ Used})Seligram one-pool example: OH rate = $4,713k / $3,260k DL = 145% of DL. Product cost = DL + 1.45 x DL.
TDABC example (BCH surgeon): $522,720 / 87,120 min = $6.00 per minute. Plagiocephaly uses 18 surgeon-minutes = $108 of surgeon cost.
Hospitals use TDABC to discover that procedures billed at the same charge actually consume vastly different resources, allowing them to negotiate reimbursement rates based on true costs. Manufacturers use ABC to stop cross-subsidising low-volume custom products with revenue from high-volume standard ones.
Students often think adding more cost pools always improves the system. There is a trade-off: more pools increase accuracy but also increase the cost and complexity of maintaining the system. The optimal system balances accuracy against redesign cost.
Students sometimes assume that undercosted products are always the small, complex ones. The direction of the distortion depends on whether the product consumes more or less of the driver than the average. A labour-intensive product in a capital-intensive plant can be overcosted, and vice versa.
Students confuse the death spiral with simple volume decline. The death spiral is specifically caused by allocating fixed costs over actual (declining) volume, inflating unit costs, and triggering price increases or product elimination that further reduces volume.
Students sometimes think joint costs can be meaningfully split between joint products. They cannot. Joint cost allocations are arbitrary and should not drive individual product decisions.
Be able to compute product costs under one-pool, two-pool, and three-pool systems and explain why the numbers differ.
Know the four steps of ABC and be able to apply them to a new scenario.
Understand that the direction of overcosting/undercosting depends on the relationship between a product's resource consumption pattern and the allocation driver used.
TDABC calculations: cost per minute x minutes used. Expect to compare TDABC costs against RCC or traditional costs.
Practical capacity vs actual volume as the denominator: know why practical capacity prevents the death spiral.
Joint costs: know the terminology (split-off point, main product, byproduct) and understand that joint cost allocation is arbitrary.
True or false: ABC uses volume-based cost drivers exclusively. (False: ABC uses drivers causally related to each activity, which may or may not be volume-based.)
Fill in the blank: the death spiral occurs when fixed costs are allocated over ______ volume rather than practical capacity. (actual)
True or false: in the Seligram case, the three-pool system always yields lower product costs than the one-pool system. (False: it redistributes costs, some products go up, others go down.)
Fill in the blank: TDABC computes a cost per ______ for each resource type. (minute)
True or false: joint costs can be meaningfully allocated to individual joint products for decision-making purposes. (False)
Q: ETO's existing system allocates $4,713k of overhead using direct labour dollars of $3,260k. What is the overhead rate?
A: $4,713k / $3,260k = 145% of direct labour dollars (or $1.45 per $1 of DL).
Q: Under ABC, Wilkerson's flow controllers see their costs increase relative to the old system. Why?
A: Flow controllers are low-volume, highly customised products. They consume disproportionate batch-level activity (setups, production runs) and product-support activity (engineering). The old system, using a single volume-based driver, spread these costs evenly, undercosting flow controllers and overcosting high-volume valves.
Q: A surgeon costs $522,720 per year and works 87,120 productive minutes. A procedure uses 18 minutes of surgeon time, 8 minutes of ASR time ($1.00/min), 23 minutes of nursing time ($1.50/min), and 5 minutes of clinical assistant time ($0.80/min). What is the TDABC cost?
A: Surgeon: 18 x $6.00 = $108. ASR: 8 x $1.00 = $8. Nurse: 23 x $1.50 = $34.50. CA: 5 x $0.80 = $4.00. Total = $154.50.
Q: Why does Anagene's overhead rate fluctuate from period to period?
A: Because Anagene calculates the rate using actual (budgeted) volume as the denominator. When volume drops, the same fixed overhead is divided by fewer units, inflating the rate. Using practical capacity as the denominator would stabilise it.
Q: Should Wendy's include the full cost of beef in its chili cost analysis?
A: No. The beef used in chili is overcooked meat that would otherwise be discarded. Its opportunity cost is near zero. Assigning the full cost of beef overstates chili's true incremental cost. Moreover, hamburgers and chili are interdependent products whose profitability should be assessed together.
Cost allocation builds on the cost behaviour foundations in Doc 1 (variable vs fixed, cost hierarchy). It feeds directly into pricing and strategy decisions (covered in Doc 3, the pricing and strategy cases), where accurate product costs are essential for setting prices and choosing product mixes. Variance analysis (Doc 4) compares actual costs against standard costs, which are only as good as the cost system that produced them.
Related Terms / Search Tags
cost allocation, cost pool, cost driver, allocation base, overhead rate, traditional costing, two-stage cost system, activity-based costing, ABC, time-driven ABC, TDABC, overcosting, undercosting, cross-subsidy, product diversity, death spiral, theoretical capacity, practical capacity, Seligram, ETO, Wilkerson, Anagene, Boston Children's Hospital, BCH, RCC, ratio of cost to charges, joint costs, joint products, split-off point, byproduct, main product, Wendy's Chili, cost per minute, homogeneous cost pool