Traditional Absorption Costing and Activity-Based Costing

Introduction

Production costing converts resource consumption into information about product, process, customer and organisational cost. In manufacturing, direct materials and direct labour can often be traced economically to individual jobs. Factory overheads—such as depreciation, production supervision, maintenance, quality assurance, utilities, material handling and production scheduling—are more difficult to trace. Costing systems therefore use allocation rules to assign indirect costs to products.

Two important approaches are traditional absorption costing and activity-based costing (ABC). Traditional absorption costing usually assigns overhead through one plant-wide rate or several departmental rates. ABC assigns costs first to activities and then to products using cost drivers that reflect the causes of resource consumption. Both systems can be useful, but they answer different managerial needs and may produce materially different product costs.

This chapter develops the concepts, calculations, interpretation and managerial applications of both systems. It also explains under- and over-absorption, the design of ABC systems, activity-based management (ABM), limitations and implementation governance.

Learning objectives

After studying this chapter, you should be able to:

1. Explain the purpose and structure of absorption costing.
2. Calculate plant-wide and departmental overhead absorption rates.
3. Determine and interpret under-absorption and over-absorption.
4. Design ABC systems using activities, cost pools and cost drivers.
5. Compare traditional costing and ABC information.
6. Use ABC and ABM to support pricing, process improvement and strategic decisions.
7. Evaluate limitations, behavioural consequences and implementation risks.

1. Cost classification and the logic of absorption costing

A product cost normally includes direct materials, direct labour and manufacturing overhead. Selling and distribution costs are generally period costs for financial reporting, although they may be included in broader customer or life-cycle analyses.

Direct cost is economically traceable to a cost object, such as a product, job or customer. Indirect cost supports several cost objects and must be allocated or apportioned. A cost object is anything for which a separate cost is required.

Absorption costing assigns all manufacturing overhead to units produced. The central logic is:

An overhead rate is calculated before or during the accounting period:

The activity level may be direct labour hours, machine hours, direct labour cost, units or another appropriate base. The rate has units, such as currency per machine hour or currency per direct labour hour.

For a job or product:

The method assumes that the selected activity base is a reasonable proxy for the consumption of overhead resources. It is most defensible where products are relatively homogeneous, overhead is modest relative to direct costs, and the chosen base has a stable cause-and-effect relationship with overhead.

Plant-wide overhead rates

A plant-wide rate uses one cost pool and one allocation base for the whole factory:

Worked example 1. A factory budgets overhead of £720,000 and 24,000 machine hours. The plant-wide rate is:

Job P uses 80 machine hours and incurs direct materials of £4,500 and direct labour of £1,600. Its absorbed overhead is £2,400 (80 × £30), and its production cost is:

The plant-wide approach is simple and inexpensive. However, it may distort costs when departments consume resources differently. A labour-intensive assembly department and an automated machining department should not necessarily receive overhead using the same driver.

Departmental overhead rates

Departmental costing creates separate cost pools and rates, commonly for machining, fabrication, assembly and finishing:

where subscript j identifies a department. A job’s total absorbed overhead is:

Worked example 2. Machining budgets £480,000 and 12,000 machine hours; assembly budgets £240,000 and 16,000 labour hours. The rates are £40 per machine hour and £15 per labour hour. Job Q uses 30 machine hours and 50 assembly labour hours:

• Machining overhead: 30 × £40 = £1,200
• Assembly overhead: 50 × £15 = £750
• Total absorbed overhead: £1,950

Departmental rates normally improve accuracy because they recognise differences in technology and resource structure. They still aggregate many activities within each department, so they may remain inaccurate when products differ substantially in batch size, complexity, inspection requirements or scheduling effort.

2. Under- and over-absorption of overhead

Predetermined rates use budgeted overhead and budgeted activity, whereas actual overhead and actual activity are known only after operations occur. Consequently, absorbed overhead may differ from actual overhead.

If the result is positive, overhead is over-absorbed. If negative, it is under-absorbed.

Equivalent presentation:

Under this second convention, a positive amount means under-absorption. Organisations must state their sign convention clearly.

Worked example 3. A department has a rate of £18 per labour hour. Actual labour hours are 10,000, so absorbed overhead is £180,000. Actual overhead is £192,000:

Thus overhead is under-absorbed by £12,000. Possible causes include higher energy prices, unplanned maintenance, overtime premiums, lower-than-expected activity, inefficient use of resources or an unrealistic budget.

Over-absorption may result from lower actual costs, higher activity, improved efficiency or an overly conservative budget. It does not automatically mean that operations performed well; high activity can increase absorption even while total costs rise.

At period end, the variance may be written off to cost of sales, allocated between work in progress, finished goods and cost of sales, or investigated through variance analysis. The treatment depends on the reporting framework, materiality and organisational policy. For managerial control, the more important task is diagnosis rather than merely closing the accounting balance.

Capacity and rate sensitivity

A rate based on practical capacity is often more informative than one based on an unusually low expected volume. If budgeted overhead is £600,000 and practical capacity is 20,000 machine hours, the rate is £30 per hour. If the same overhead is divided by expected use of 15,000 hours, the rate becomes £40 per hour. The latter may load idle-capacity cost onto products and make prices appear uncompetitive. Capacity cost analysis should therefore distinguish the cost of resources supplied from the cost of resources actually used.

3. Activity-based costing

ABC responds to the observation that products consume activities, and activities consume resources. It does not assume that one volume measure explains all overhead. Instead, it identifies significant activities, groups their costs into homogeneous cost pools, selects cost drivers and assigns costs according to driver usage.

A typical ABC chain is:

Resources → Activities → Cost pools → Cost drivers → Products/customers

Important terms are:

• Activity: A task or related group of tasks, such as setting up a machine, processing a purchase order or inspecting a batch.
• Cost pool: Costs accumulated for an activity or group of homogeneous activities.
• Cost driver: A measurable factor that causes or significantly influences activity cost.
• Resource driver: A basis for assigning resource costs to activities, such as staff time or floor area.
• Activity driver: A basis for assigning activity-pool cost to cost objects, such as number of setups or inspections.
• Homogeneous cost pool: A pool in which costs have broadly similar driver relationships.

ABC commonly distinguishes four activity levels:

The classification is analytical rather than absolute. An inspection may be batch-level in one factory and unit-level in another. The driver must reflect the actual process.

ABC calculation

For activity pool i:

For product p:

The rate’s unit must match the driver: for example, £250 per setup or £12 per purchase order. The method assumes that costs in each pool are sufficiently homogeneous and that driver quantities are measured reliably.

Worked example 4. A factory produces Standard and Custom products. Its overhead is analysed as follows:

Standard uses 8,000 machine hours, 100 setups, 400 inspections and 600 purchase orders. Custom uses 7,000 machine hours, 500 setups, 800 inspections and 1,400 purchase orders.

Standard receives:

Custom receives:

Total assigned overhead is £700,000, equal to total pool cost. Custom consumes fewer machine hours but many more setups, inspections and purchase orders; ABC therefore reveals its greater overhead complexity.

4. Comparing traditional costing and ABC

Traditional costing tends to overcost high-volume, simple products and undercost low-volume, complex products when a single volume driver is used. This phenomenon is often called cost cross-subsidisation. The high-volume product absorbs too much overhead because the system ignores the complexity caused by the low-volume product.

Suppose the same £700,000 overhead were allocated only by machine hours. The plant rate would be £46.67 per machine hour. Standard would receive approximately £373,333 and Custom £326,667. Compared with ABC, Standard is overcosted by approximately £103,333 and Custom undercosted by approximately £103,333. The difference could affect prices, product mix, outsourcing and process priorities.

ABC does not guarantee truth. It provides a more detailed model of resource consumption. If driver relationships are weak, data are poor or pools are arbitrarily designed, ABC can create false precision. The relevant question is not whether ABC is more sophisticated, but whether the additional information improves decisions enough to justify its cost.

Traditional absorption costing remains important for inventory valuation and external reporting in many jurisdictions. ABC is primarily a managerial tool, although ABC information may support operational and strategic analysis.

5. Designing and implementing an ABC system

An effective ABC project should be designed around decisions, not around the desire to measure everything. A practical sequence is:

1. Define the purpose and cost objects: products, services, customers, channels or processes.
2. Map major processes and identify activities.
3. Gather resource costs from the general ledger and assign them to activities using resource drivers.
4. Combine only activities with similar cost behaviour into a pool.
5. Select drivers that have a credible causal or operational relationship with activity cost.
6. Measure practical driver capacity and actual or expected usage.
7. Calculate rates and assign costs.
8. Validate results with operational managers and reconcile assigned costs to the ledger.
9. Test decisions using sensitivity analysis.
10. Establish ownership, review frequency and data controls.

Driver selection involves a trade-off among causal validity, measurement cost and behavioural acceptability. A driver should be understandable, available at reasonable cost, resistant to manipulation and sufficiently stable for the intended decision.

ABC may use interviews and time estimates, but these should be tested against transaction records, machine data, enterprise-resource-planning systems and process observations. Time-driven ABC is a related approach that estimates the cost per unit of supplied capacity and the time required for transactions:

For a transaction:

Time-driven ABC can reduce the number of separate interviews and update costs when process times change. Its estimates nevertheless require validation, especially where waiting, rework or unused capacity is significant.

6. Activity-based management

ABM uses cost and operational information to improve activities and decisions. It has two complementary dimensions:

• Operational ABM: improve efficiency, quality, cycle time and capacity utilisation.
• Strategic ABM: reconsider product mix, customer profitability, supplier relationships, channels and process design.

A cost pool identifies where resources are consumed; ABM asks why the activity exists and whether it creates value. Activities may be classified as value-added, business-value-added or non-value-added. For example, transforming material may add customer value, while avoidable rework, duplicate data entry and waiting may not.

ABM should not interpret every non-value-added activity as immediately eliminable. Inspection may not add direct customer value, but it may be necessary under current process capability or regulatory requirements. Improvement should address the underlying cause rather than simply removing a control.

Relevant ABM measures include setup time, first-pass yield, purchase orders per product, engineering changes, delivery reliability, defect cost and unused capacity. Financial measures should be combined with quality, safety, employee and customer indicators.

7. Applied case: Meridian Components Ltd.

Background

Meridian Components Ltd. produces two families of industrial valves: the high-volume M valve and the low-volume X valve. M is produced in long runs with few design changes. X requires small batches, frequent setups, additional inspections and many engineering discussions. Management currently applies all £1.2 million of factory overhead using machine hours.

Problem

M represents 80% of machine hours but only 55% of sales revenue; X represents 20% of machine hours but 45% of sales revenue. The sales team believes that X is highly profitable because its selling price is high. Manufacturing managers argue that X disrupts the factory and consumes disproportionate support resources.

Analysis

Management identifies three pools:

Annual usage is:

Under ABC, M receives:

X receives:

The traditional machine-hour rate is £50 per hour (£1.2 million ÷ 24,000). It assigns £960,000 to M and £240,000 to X. Thus traditional costing overstates M’s overhead by £270,000 and understates X’s by £270,000.

Approach and outcome

Meridian does not immediately change prices. It first validates the driver data, examines whether X’s engineering requirements are customer-driven, and separates avoidable from unavoidable support cost. It then considers larger X batch sizes, modular product design, setup reduction and a surcharge for genuinely customised orders. The company also reports unused capacity separately so that managers do not treat unavoidable facility cost as a product penalty.

Learning points

The case demonstrates that overhead distortion can arise even when a traditional base appears technically related to production. ABC makes complexity visible, but managerial action requires process analysis, customer communication and a distinction between avoidable and unavoidable cost. Cost information should inform—not mechanically determine—commercial decisions.

8. Limitations, governance and contemporary practice

ABC can be expensive to implement and maintain. Large numbers of pools may overwhelm users, while too few pools recreate the distortion the system was intended to correct. Allocating facility-sustaining costs remains difficult because no product may cause them in the short term. Common data problems include inconsistent definitions, inaccurate driver counts, duplicated transactions and poor treatment of idle capacity.

Cost allocation also has behavioural consequences. Managers may reject profitable work if fully allocated cost is confused with relevant incremental cost. A product can appear loss-making after receiving unavoidable fixed overhead while still contributing positively to short-run cash flow. Pricing and outsourcing decisions should therefore distinguish full cost, avoidable cost, opportunity cost and strategic value.

Governance requires documented definitions, reconciliations to the general ledger, version control, review of driver rates, audit trails and clear accountability. Allocation methods should not be manipulated to reward a department or conceal inefficiency. Employee time data should be collected transparently and proportionately, with appropriate privacy safeguards.

Modern manufacturing execution systems, enterprise-resource-planning systems, sensors and process-mining tools can automate driver measurement. AI may help classify transactions, detect anomalies and estimate process times, but it does not establish causality by itself. Models should be explainable, monitored for bias and reviewed by process experts. Automated data collection must not turn employee monitoring into disproportionate surveillance.

Best practice is therefore contingent: retain a simple plant-wide or departmental system where it is sufficiently accurate; use ABC where product, customer or process diversity makes distortion material; and combine cost information with operational, quality, capacity and strategic evidence.

Chapter Summary

Traditional absorption costing assigns manufacturing overhead to products through a plant-wide or departmental rate. The general rate is budgeted overhead divided by budgeted activity, and absorbed overhead equals the rate multiplied by activity used. Differences between actual and absorbed overhead create under- or over-absorption and should be investigated rather than treated only as an accounting adjustment.

ABC recognises that products consume activities and activities consume resources. It uses activity cost pools and drivers such as setups, inspections, purchase orders and engineering events. ABC can reveal cross-subsidisation between high-volume simple products and low-volume complex products, although its benefits must exceed its measurement and maintenance costs. ABM extends ABC by improving processes, eliminating avoidable activity and supporting strategic decisions. Sound practice requires causal drivers, reliable data, capacity analysis, governance and careful interpretation of relevant costs.

Key Terms

Cost object: Anything for which a separate cost is measured.

Direct cost: A cost economically traceable to a cost object.

Indirect cost: A cost requiring allocation or apportionment.

Manufacturing overhead: Indirect production cost other than direct materials and direct labour.

Absorption costing: Assigning manufacturing overhead to products.

Plant-wide rate: One overhead rate for the whole factory.

Departmental rate: An overhead rate calculated separately for a department.

Predetermined overhead rate: Budgeted overhead divided by budgeted activity.

Absorbed overhead: Overhead assigned using an absorption rate.

Under-absorption: Actual overhead exceeds absorbed overhead.

Over-absorption: Absorbed overhead exceeds actual overhead.

Activity-based costing: Costing that assigns resource costs through activities and drivers.

Cost pool: Accumulated cost for a homogeneous activity or group of activities.

Cost driver: A factor that causes or influences activity cost.

Resource driver: A basis for assigning resources to activities.

Activity driver: A basis for assigning activity costs to cost objects.

Cost cross-subsidisation: Overcosting one product while undercosting another.

Activity-based management: Using activity information to improve operations and strategy.

Practical capacity: Usable capacity after allowing for unavoidable interruptions.

Time-driven ABC: ABC using capacity cost rates and estimated transaction time.

Review Questions

Short Answer Questions

1. What is a predetermined overhead rate, and why is it used?
2. Distinguish a plant-wide rate from a departmental rate.
3. Define under-absorption and identify two possible causes.
4. What is a cost driver in ABC?
5. Distinguish operational ABM from strategic ABM.

Long Answer Questions

1. Explain the assumptions, strengths and limitations of traditional absorption costing.
2. Describe the stages involved in designing an ABC system.
3. Compare plant-wide, departmental and activity-based costing in terms of accuracy, cost and usefulness.
4. Explain how unused capacity affects overhead rates and managerial decisions.
5. Discuss the governance, behavioural and ethical risks of automated cost allocation.

Analytical / Application Questions

1. A factory budgets overhead of £450,000 and 15,000 machine hours. A product uses 120 machine hours, direct materials of £2,800 and direct labour of £900. Calculate the overhead rate and total production cost before selling costs.
2. A company has setup costs of £240,000 for 800 setups and inspection costs of £150,000 for 1,000 inspections. Product A uses 40 setups and 60 inspections; Product B uses 160 setups and 140 inspections. Calculate each product’s assigned setup and inspection cost and explain the managerial implication.
3. A firm’s traditional system allocates overhead only by machine hours, while its ABC analysis shows that a low-volume product consumes most engineering-change orders. Recommend whether management should change the price immediately, and identify the additional information required before making the decision.

Multiple Choice Questions with Answers

Question 1

A factory budgets £600,000 of overhead and 20,000 machine hours. What is the predetermined overhead rate?

A. £20 per machine hour

B. £30 per machine hour

C. £40 per machine hour

D. £50 per machine hour

Correct Answer: B. £30 per machine hour

Question 2

If absorbed overhead is £180,000 and actual overhead is £192,000, the result is:

A. £12,000 over-absorption

B. £12,000 under-absorption

C. £372,000 under-absorption

D. No variance

Correct Answer: B. £12,000 under-absorption

Question 3

Which is most likely a batch-level activity?

A. Machining each unit

B. Factory security

C. Setting up a machine for a production run

D. Product design across its entire life

Correct Answer: C. Setting up a machine for a production run

Question 4

An ABC pool costs £90,000 and contains 300 setups. What is its driver rate?

A. £30 per setup

B. £270 per setup

C. £300 per setup

D. £900 per setup

Correct Answer: C. £300 per setup

Question 5

A product uses 12 setups at £300 per setup and 50 inspections at £20 per inspection. What overhead is assigned from these pools?

A. £3,600

B. £4,000

C. £4,600

D. £6,000

Correct Answer: C. £4,600

Question 6

Traditional costing is most likely to undercost a product that is:

A. High-volume and standardised

B. Low-volume and highly complex

C. Produced in long runs with few inspections

D. Using very few engineering changes

Correct Answer: B. Low-volume and highly complex

Question 7

Which statement best describes a homogeneous cost pool?

A. It contains all factory costs regardless of cause

B. Its costs have broadly similar driver relationships

C. It includes only direct material costs

D. It must use machine hours as its driver

Correct Answer: B. Its costs have broadly similar driver relationships

Question 8

If actual activity is lower than expected while fixed overhead remains unchanged, the absorption rate may produce:

A. Under-absorption

B. Over-absorption necessarily

C. No possible variance

D. A direct-material variance only

Correct Answer: A. Under-absorption

Question 9

The capacity cost rate in time-driven ABC is calculated as:

A. Actual output divided by selling price

B. Cost of supplied capacity divided by practical capacity supplied

C. Total sales divided by labour hours

D. Budgeted profit divided by units sold

Correct Answer: B. Cost of supplied capacity divided by practical capacity supplied

Question 10

Which is an example of strategic ABM?

A. Reducing duplicate data entry within a process

B. Measuring daily setup time

C. Evaluating whether a customer segment remains profitable

D. Recording factory electricity usage

Correct Answer: C. Evaluating whether a customer segment remains profitable