Source: Horngren's Cost Accounting: A Managerial Emphasis, 16th ed. (Datar/Rajan), Chapters 1–3
Tags: cost accounting, management accounting, cost behaviour, variable cost, fixed cost, mixed cost, cost object, direct cost, indirect cost, cost driver, relevant range, ACCTMIS 3300, Ohio State, Fisher College of Business, Horngren 16th edition
Difficulty: Foundational | Prerequisites: Introductory financial accounting (ACCTMIS 2200 or equivalent). A working knowledge of Excel and basic formula use is expected.
Cost accounting sits at the intersection of financial reporting and management decision-making. Where financial accounting looks outward (investors, regulators), cost accounting looks inward: how much does it cost to make a product, run a department, or serve a customer, and what should management do about it?
This first unit lays the vocabulary and behavioural models that every later topic depends on. If you cannot classify a cost correctly or predict how it moves when volume changes, the budgeting, variance analysis, and allocation methods in Parts 2 and 3 will not make sense.
The material here applies well beyond manufacturing. Any organisation that allocates shared costs, builds a budget, or evaluates performance uses these concepts, whether it is a hospital, a consultancy, or a tech firm.
Cost accounting measures, analyses, and reports costs to help managers plan, control, and make decisions. Costs are classified by their behaviour (fixed, variable, mixed), by their traceability to a cost object (direct or indirect), and by their relevance to a particular decision. Getting these classifications right is the foundation for everything else in the course.
Cost object
Anything for which a separate measurement of cost is wanted: a product, a service, a department, a customer, a project. The cost object is always the starting point; you cannot classify a cost as direct or indirect until you have named the cost object.
In simple terms, it is the "thing" you are trying to figure out the cost of.
Direct cost
A cost that can be traced to a specific cost object in an economically feasible way. The tracing must be physically observable or clearly linked.
Think of it as a cost you can point to and say "that went to this product and only this product," such as the timber in a table or the wages of a worker who assembles only one product line.
Indirect cost (overhead)
A cost related to the cost object but not traceable to it in an economically feasible way. These costs are allocated rather than traced.
Think of it as the rent for a factory that produces five different products. The rent benefits all five, but you cannot measure exactly how much of the rent "belongs" to each one without some allocation method.
Cost driver
Any factor that causes a change in total cost. Volume of output is the most common cost driver, but machine hours, number of setups, and number of purchase orders are all cost drivers too.
In simple terms, it is the activity or variable that makes costs go up or down.
Variable cost
A cost that changes in total in proportion to changes in the level of activity (the cost driver). Per-unit variable cost stays constant within the relevant range.
Think of it as the fabric used to make shirts: double the shirts, double the fabric cost.
Fixed cost
A cost that does not change in total over a given period, regardless of changes in the level of activity within the relevant range. Per-unit fixed cost decreases as volume increases (spreading effect).
Think of it as the monthly lease on a factory. Whether you produce 1,000 units or 10,000 units that month, the lease payment is the same.
Mixed cost (semi-variable cost)
A cost that contains both a fixed component and a variable component. Total mixed cost = fixed portion + (variable rate per unit x activity level).
Think of a mobile phone contract with a flat monthly fee plus a per-minute charge for calls over a threshold.
Relevant range
The band of normal activity within which the assumed cost-behaviour patterns (fixed stays fixed, variable stays proportional) hold true. Outside this range, costs may jump or the per-unit rate may shift.
In simple terms, cost behaviour assumptions are only reliable within the volume levels the organisation normally operates at.
Cost behaviour
How a cost responds to changes in the level of a related activity. The three primary patterns are variable, fixed, and mixed. Correct identification of cost behaviour is essential for budgeting, CVP analysis, and variance analysis.
Period cost
A cost that is expensed in the accounting period in which it is incurred, rather than being attached to a product. Selling costs and general administrative costs are the classic examples.
Product cost (inventoriable cost)
A cost that is attached to units of output and sits in inventory (as an asset) until the goods are sold, at which point it becomes cost of goods sold. Under absorption costing, product costs include direct materials, direct labour, and manufacturing overhead.
Prime cost
Direct materials + direct labour. The costs most directly and obviously traceable to the product.
Conversion cost
Direct labour + manufacturing overhead. The costs of converting raw materials into finished goods.
Sunk cost
A cost that has already been incurred and cannot be recovered regardless of any future decision. Sunk costs are irrelevant to decision-making, though people routinely (and incorrectly) factor them in.
Opportunity cost
The benefit forgone by choosing one alternative over the next best alternative. Opportunity costs do not appear in accounting records but are critical in decision analysis.
Controllable cost
A cost that a specific manager can influence within a given time period. What is controllable depends on the level of authority and the time horizon.
Financial accounting serves external users (investors, creditors, regulators) and follows GAAP or IFRS.
Management accounting serves internal users (managers at all levels) and has no required format or standard.
Cost accounting overlaps both: it supplies inventory valuations for financial statements and cost data for internal decisions.
Management accounting is forward-looking (budgets, forecasts), while financial accounting is primarily historical.
There is no legal requirement for management accounting reports, so their design is driven purely by usefulness.
Cost measurement: determining what it costs to produce a unit, run a department, or serve a customer.
Cost control: monitoring costs against budgets or standards and investigating variances.
Decision support: providing relevant cost information for pricing, outsourcing, product-mix, and capital investment decisions.
Variable costs move in total with the cost driver. Examples: direct materials, sales commissions, shipping costs per unit.
Per-unit variable cost is constant within the relevant range.
Total variable cost = variable cost per unit x number of units.
Fixed costs stay constant in total within the relevant range. Examples: rent, insurance, salaried manager pay.
Per-unit fixed cost decreases as volume rises (this is the "spreading" or "absorption" effect).
Total fixed cost is unchanged until the firm moves outside its relevant range (e.g., leases a second factory).
Mixed costs combine both behaviours. The high-low method and regression analysis are common techniques for splitting the fixed and variable components.
Select the highest and lowest activity levels from the data set.
Variable cost per unit = (Cost at high activity – Cost at low activity) / (High activity – Low activity).
Fixed cost = Total cost at either point – (Variable cost per unit x Activity at that point).
The high-low method is simple but uses only two data points, so it can be distorted by outliers.
Direct costs are traced to the cost object. The link is physically observable.
Indirect costs are allocated to the cost object using a cost-allocation base (e.g., machine hours, labour hours).
Whether a cost is direct or indirect depends entirely on the cost object chosen. Factory rent is indirect to a product but direct to the factory as a whole.
Direct materials: raw materials that become part of the finished product and are economically traceable to it.
Direct labour: wages of workers who physically convert materials into the finished product.
Manufacturing overhead (MOH): all other manufacturing costs that are not direct materials or direct labour. Includes indirect materials, indirect labour, utilities, depreciation on factory equipment, and factory rent.
Prime cost = direct materials + direct labour.
Conversion cost = direct labour + manufacturing overhead.
Costs flow through three inventory accounts: Raw Materials Inventory, Work-in-Process (WIP) Inventory, and Finished Goods Inventory.
When raw materials are issued to production, they move from Raw Materials to WIP.
As products are completed, costs move from WIP to Finished Goods.
When products are sold, costs move from Finished Goods to Cost of Goods Sold on the income statement.
A relevant cost is a future cost that differs between alternatives.
Sunk costs are never relevant (the money is already spent).
Opportunity costs are always relevant (the benefit of the next-best option you are giving up).
Allocated fixed costs that do not change between alternatives are not relevant, even if they appear in a cost report.
Total cost equation:
Total Cost = Total Fixed Costs + (Variable Cost per Unit x Number of Units)
High-low method:
Variable Cost per Unit = (Cost_high – Cost_low) / (Activity_high – Activity_low)
Fixed Cost = Total Cost at either point – (Variable Cost per Unit x Activity at that point)
Prime cost:
Prime Cost = Direct Materials + Direct Labour
Conversion cost:
Conversion Cost = Direct Labour + Manufacturing Overhead
Cost of Goods Manufactured (COGM):
COGM = Beginning WIP + Total Manufacturing Costs Incurred – Ending WIP
Cost of Goods Sold (COGS):
COGS = Beginning Finished Goods + COGM – Ending Finished Goods
These cost classifications are not abstract. When a ride-sharing company decides its per-ride pricing, it needs to know which costs are variable (fuel, driver payout per ride) and which are fixed (app server costs, salaried engineers). Misclassifying a fixed cost as variable, or vice versa, leads to incorrect break-even points and mispriced services.
In manufacturing, the flow of costs through Raw Materials, WIP, and Finished Goods is how companies value their inventory on the balance sheet. Get the overhead allocation wrong and you misstate inventory, which misstates profit.
"Direct" means the same as "variable." It does not. Direct and indirect describe traceability to a cost object. Variable and fixed describe behaviour relative to activity. A factory supervisor's salary might be direct to the factory (you can trace it there) but fixed (it does not change with the number of units produced).
"Fixed costs are always fixed." Fixed costs are fixed only within the relevant range and only in the short run. Over a longer horizon, almost every cost becomes variable.
"Sunk costs should be recovered." Sunk costs are gone. Basing future decisions on recovering sunk costs leads to poor choices (the "throwing good money after bad" problem).
"Higher volume always lowers cost per unit." It lowers fixed cost per unit (the spreading effect), but variable cost per unit stays the same. And if volume exceeds the relevant range, fixed costs may step up.
⚠️ Be able to classify any cost as direct or indirect given a stated cost object, and as variable or fixed given a stated cost driver. The exam will change the cost object or driver between questions to test whether you understand the distinction.
⚠️ The high-low method is a very common calculation question. Watch for data sets that include an outlier at the high or low end; if the question says to exclude outliers, do so before applying the formula.
⚠️ Know the cost-flow sequence (Raw Materials → WIP → Finished Goods → COGS) and be able to compute COGM and COGS from a set of beginning and ending balances.
⚠️ Understand why sunk costs and allocated fixed costs that do not differ between alternatives are irrelevant. The exam will include tempting wrong answers that factor them in.
True or false: A cost that is direct to a product must also be variable.
Fill in the blank: The band of activity within which cost-behaviour assumptions hold is called the __________.
True or false: Opportunity costs appear in the general ledger.
Fill in the blank: Prime cost equals direct materials plus __________.
True or false: If a factory's monthly rent is £20,000 and it produces 10,000 units this month instead of 5,000, the total rent cost doubles.
Answers: 1. False. 2. Relevant range. 3. False. 4. Direct labour. 5. False (total rent is unchanged; per-unit rent halves).
Q: A company produces wooden desks. It pays a flat monthly salary of £4,500 to a factory supervisor and £12 per desk in raw timber. Classify each cost by behaviour and by traceability to individual desks.
A: The supervisor's salary is a fixed cost (does not change with the number of desks) and an indirect cost (the supervisor oversees all production, not traceable to one desk). The timber is a variable cost (total cost rises proportionally with volume) and a direct cost (traceable to each desk).
Q: Using the high-low method, a company's highest activity was 8,000 units at a total cost of £58,000, and its lowest activity was 3,000 units at a total cost of £38,000. What are the variable cost per unit and the total fixed cost?
A: Variable cost per unit = (£58,000 – £38,000) / (8,000 – 3,000) = £20,000 / 5,000 = £4 per unit. Fixed cost = £58,000 – (£4 x 8,000) = £58,000 – £32,000 = £26,000. (Or check: £38,000 – (£4 x 3,000) = £38,000 – £12,000 = £26,000.)
Q: A firm spent £100,000 developing a prototype that did not work. It can now spend £50,000 to redesign or £30,000 to scrap it and start over. What is the sunk cost, and which option's cost is relevant?
A: The £100,000 already spent is a sunk cost and is irrelevant to the decision. The relevant costs are £50,000 (redesign) and £30,000 (scrap and restart). The decision should be based on which alternative produces the better future outcome, ignoring the £100,000.
Q: Define conversion cost and explain why it groups direct labour with manufacturing overhead.
A: Conversion cost = direct labour + manufacturing overhead. It groups these because both represent the costs of converting raw materials into finished products. Direct materials are excluded because they are the input being converted, not the cost of conversion itself.
This material connects directly to Part 2 (Costing Systems and Cost-Allocation Methods), where the distinction between direct and indirect costs determines how overhead is applied to products using job-order or process costing. Understanding cost behaviour is also prerequisite to CVP (cost-volume-profit) analysis and to the flexible budgeting and variance analysis covered in Part 3. If you are unclear on variable vs. fixed or direct vs. indirect, those later topics will be significantly harder.
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