Difficulty: Intermediate | Prerequisites: Introductory financial accounting, basic understanding of cost behaviour (fixed vs variable costs).
This material sits at the heart of a managerial accounting course. It covers how companies assign indirect costs to products, why the method they choose matters, and what goes wrong when the method is too simple. If you understand cost behaviour (fixed, variable, mixed) and can read an income statement, you are ready. The concepts here feed directly into product-mix decisions, pricing strategy, and capacity management, all of which appear later in the course.
Traditional costing systems allocate all overhead using a single volume-based driver (typically direct labour dollars or hours). Activity-based costing (ABC) uses multiple cost pools, each with its own driver chosen to reflect what causes the cost. ABC tends to reveal that low-volume, complex products are undercosted and high-volume, simple products are overcosted under the traditional system, which can lead to poor pricing and product-mix decisions.
Cost object
Anything to which costs are assigned: a product, a department, a customer, a project. All cost classification (direct/indirect, fixed/variable) is relative to the chosen cost object.
In simple terms: whatever you are trying to figure out the cost of.
Direct cost
A cost that can be traced to a single cost object in an economically feasible way. Direct materials and direct labour are the classic examples.
Think of it as: if you can point at the product and say "that cost went there and nowhere else," it is direct.
Indirect cost (overhead)
A cost that cannot be traced to a single cost object in an economically feasible way and must therefore be allocated. Factory rent, depreciation on shared equipment, and supervisory salaries are typical.
In simple terms: costs shared across products that need a formula to divvy up.
Cost driver
A factor that causes or is closely correlated with a cost. Used as the basis for allocating indirect costs. A good cost driver reflects a cause-and-effect relationship between the activity and the cost.
Think of it as: the thing you count to decide how much overhead each product should absorb.
Cost pool
A grouping of individual indirect costs that share a single cost driver. In a traditional system there is often one cost pool; in ABC there are several.
In simple terms: a bucket of overhead costs that all get spread using the same allocation base.
Traditional (volume-based) costing
A cost system that allocates all manufacturing overhead using a single volume-related driver, most commonly direct labour dollars or direct labour hours.
Think of it as: one big pot of overhead, spread by one measure of volume.
Activity-based costing (ABC)
A costing methodology that identifies the activities a firm performs, groups costs by activity, and allocates each activity's cost pool using a driver specific to that activity. Typical activity pools include setups, machine hours, inspections, and shipments.
In simple terms: overhead is split into smaller, more targeted pools, each allocated by what causes it.
Overcosting / undercosting
A product is overcosted when the costing system assigns it more indirect cost than it consumes, and undercosted when the system assigns less than it consumes. Because total overhead is fixed, overcosting one product necessarily means undercosting another.
Think of it as: a cross-subsidy, where one product quietly pays for another's overhead.
Death spiral
A feedback loop in which a firm raises prices to cover rising per-unit costs (caused by spreading fixed costs over fewer units), which drives away more volume, which raises per-unit costs further. Can occur with a single product or multiple products.
In simple terms: costs go up, prices go up, customers leave, costs go up again.
Three warning signs that a costing system is distorting product costs (from the Wilkerson case):
Competitors' prices appear unrealistically low. If your costing says a product costs more than rivals charge, the system may be overcosting it.
Complex or low-volume products show unusually high profit margins. The system may be undercosting them by spreading overhead too thinly.
You raise your price and demand does not fall. This suggests the market already sees your product as cheaper than alternatives, meaning the system may be undercosting it.
All three of these are symptoms, and they can appear simultaneously.
A traditional system lumps all overhead into one pool and allocates it using a single volume driver (e.g. direct labour dollars). This works tolerably when products consume overhead in roughly the same proportions as they consume the driver. It breaks down when products differ in complexity, batch size, or the types of activities they require.
Consider the Seligram case: the system used direct labour dollars as the sole driver for overhead. A component that required extensive testing (many labour hours) absorbed a large share of overhead, even if the overhead consumed per test was identical across components. The result: the labour-intensive component was overcosted, and the component needing little testing was undercosted.
The general pattern: under a single volume-based driver, high-volume simple products tend to be overcosted and low-volume complex products tend to be undercosted.
ABC breaks overhead into multiple cost pools, each allocated by a driver that reflects the activity's consumption pattern:
Setup costs allocated by number of setups
Machine-related costs allocated by machine hours
Packing costs allocated by number of shipments
Inspection costs allocated by number of inspections
By matching each pool to its causal driver, ABC captures the different ways products consume resources. A low-volume product that requires many setups per unit absorbs more setup cost, which a labour-based system would miss.
Adding a new cost pool does not always improve accuracy. If the new pool's driver is highly correlated with an existing driver, splitting it out adds complexity without improving the allocation. A cost pool improves accuracy only when it captures a meaningfully different consumption pattern.
Luzent produces Deluxe and Standard doors. Under the traditional system (direct labour dollars as driver), both products have the same direct labour cost per unit ($55), so they receive the same overhead rate per labour dollar.
Under ABC, three activity pools emerge:
Setups: Deluxe has 400 setups for 50,000 units (0.008 setups/unit); Standard has 100 setups for 400,000 units (0.00025 setups/unit). Deluxe is 32 times more setup-intensive per unit.
Machine hours: equal total hours (300,000 each), but Deluxe makes only 50,000 units (6 mh/unit) vs Standard's 400,000 units (0.75 mh/unit). Deluxe is 8 times more machine-intensive per unit.
Packing: equal shipments (50,000 each), so Deluxe at 1 shipment/unit vs Standard at 0.125 shipments/unit.
The traditional system overcosted Standard and undercosted Deluxe because it allocated overhead by labour dollars (equal per unit), ignoring that Deluxe consumes far more setups and machine hours per unit.
\text{Traditional overhead rate} = \frac{\text{Total overhead}}{\text{Total allocation base (e.g. direct labour \$)}}\text{Overhead applied per unit} = \text{Overhead rate} \times \text{Driver quantity per unit}\text{ABC cost rate for activity } i = \frac{\text{Total cost of activity } i}{\text{Total quantity of cost driver } i}\text{Total manufacturing cost per unit} = \text{DM} + \text{DL} + \sum (\text{ABC rate}_i \times \text{driver}_i \text{ per unit})\text{Profit per unit} = \text{Sales price} - \text{Total manufacturing cost per unit}For the Luzent example, the traditional overhead rate is computed as:
Total overhead = $500,000 + $44,000,000 + $5,000,000 = $49,500,000.
Total direct labour dollars = (50,000 x $55) + (400,000 x $55) = $24,750,000.
Overhead rate = $49,500,000 / $24,750,000 = $2.00 per direct labour dollar.
Overhead per unit (both products) = $55 x $2.00 = $110.00.
Note: the exam states $80 for Deluxe and $120 for Standard under the traditional system. This discrepancy arises because the exam's traditional figures were given directly (they may reflect a different overhead rate or additional cost data not shown). Use the figures given in the problem when answering exam questions, and use the ABC data to recalculate.
ABC was developed in manufacturing settings where product lines had diverged in complexity. A factory making both a simple commodity part and a custom-engineered variant on the same line is the textbook scenario: the commodity absorbs too much overhead under a volume-based system, making it look unprofitable, while the custom part looks artificially cheap.
The same logic applies in services. A bank offering both standard current accounts and complex structured-finance products will misallocate shared costs (compliance, IT, customer service) if it spreads them by a single driver like number of accounts.
Students often think a cost cannot be both direct and fixed. It can. A factory manager's salary is direct to the factory (traceable) and fixed (does not change with volume). The two classifications, direct/indirect and fixed/variable, are independent.
Students often think adding more cost pools always improves accuracy. It does not. A new pool helps only if its driver captures a consumption pattern that existing drivers miss. A redundant pool adds complexity without improving accuracy.
Students often think the death spiral requires multiple products. It does not. A single-product firm can enter a death spiral if it spreads fixed costs over declining volume and raises prices in response.
Students often think the traditional system gets total overhead wrong. It does not. Total overhead is the same under both systems. What changes is how that total is distributed across products.
Be able to calculate the overhead rate under a traditional system and under ABC, then compute profit per unit both ways. This is a bread-and-butter exam problem (see Problem 2 on the practice exam).
Know how to explain verbally, without any maths, why costs shift between products when moving from traditional to ABC. The examiners want you to identify which drivers cause which product to absorb more or less cost.
Understand that overcosting one product means undercosting another, because total overhead does not change.
The Seligram case tests whether you can identify which component is over- or undercosted when the driver does not match actual resource consumption.
The Wilkerson case tests whether you can spot the symptoms of an unreliable costing system from qualitative clues.
True or false: A cost can be both direct and fixed with respect to the same cost object. (True)
True or false: Adding a new cost pool always improves the accuracy of the costing system. (False)
True or false: Death spirals can only occur when a firm produces multiple products. (False)
Fill in the blank: Under a traditional costing system that uses direct labour dollars, a product with high machine usage but average labour usage will be ______ (overcosted / undercosted). (Undercosted)
True or false: Under ABC, total overhead allocated across all products is different from total overhead under the traditional system. (False, same total)
Q: In the Wilkerson case, which of the following appeared as symptoms of an unreliable costing system? (a) Competitors' prices appear unrealistically low, (b) Complex products show high profit margins, (c) We increase our price and demand does not fall, (d) All of the above.
A: (d) All of the above. Each is a classic red flag that the costing system is misallocating overhead.
Q: In the Seligram case, the existing system uses direct labour dollars as the sole cost driver. Component X requires extensive testing (many labour hours); component Y requires little testing. Overhead consumed per test is identical. Which component is overcosted?
A: X is overcosted and Y is undercosted. X absorbs too much overhead because it uses more labour hours, even though both components consume the same overhead per test.
Q: Explain verbally how and why Deluxe and Standard door costs change when Luzent switches from the traditional system to ABC.
A: Under the traditional system, both products receive overhead in proportion to direct labour dollars, which are equal ($55 per unit). Under ABC, three separate drivers reveal that Deluxe uses far more setups per unit (400/50,000 vs 100/400,000) and far more machine hours per unit (6 vs 0.75). These drivers shift overhead toward Deluxe and away from Standard. Deluxe's cost per unit rises; Standard's falls. The traditional system was overcosting Standard and undercosting Deluxe.
Q: A firm uses capacity-based cost drivers. If capacity increases, what happens to cost per unit and the cost of unused capacity?
A: Cost per unit declines (the rate is calculated using higher capacity in the denominator) and the cost of unused capacity increases (the gap between capacity and actual usage widens).
Q: If indirect cost rates were based on actual short-term usage, would periods of lower demand result in higher costs per unit?
A: True. With lower demand, fixed costs are spread over fewer units, so the rate per unit rises.
This material connects directly to product-mix optimisation. Once you know the true cost of each product (via ABC), you can make better decisions about which products to promote, which to discontinue, and how to price them. It also connects to capacity-cost management: the choice between using actual usage or practical capacity as the denominator in overhead rates determines whether unused-capacity costs are hidden inside product costs or reported separately.
The joint-products topic shares the same underlying question: how do you allocate shared costs, and does the allocation affect decisions? The answer in joint costing is that allocated joint costs are irrelevant to sell-or-process-further decisions, just as the choice of overhead driver should not change total overhead but does change which products look profitable.
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