Difficulty: Introductory | Prerequisites: Basic financial accounting (debits/credits, income statement structure)
This is the foundation of managerial accounting. Before you can do CVP analysis, make-or-buy decisions, or any of the applied topics in ACC 312, you need a solid grip on how costs behave when activity levels change and how they are classified. This material sits at the very start of the course for a reason: every later topic assumes you can look at a cost and immediately say whether it is fixed, variable, mixed, or step, and whether it is a product cost or a period cost. If you are coming in cold, start here.
Costs behave differently as activity changes. Some stay flat (fixed), some move in lockstep with volume (variable), some do a bit of both (mixed), and some jump in blocks (step). Knowing which is which drives every planning, budgeting, and decision-making tool in this course.
Variable cost
A cost that changes in direct proportion to changes in the activity level. Total variable cost rises or falls with volume, but the variable cost per unit stays constant within the relevant range. In simple terms, the more you make, the more you spend, at a steady rate per unit.
Fixed cost
A cost that remains constant in total regardless of changes in the activity level, within the relevant range. Fixed cost per unit, however, drops as volume rises (the total is spread over more units). Think of it as rent on a factory: the landlord charges the same whether you make 100 units or 10,000.
Committed fixed cost
A fixed cost that arises from long-term decisions and cannot easily be changed in the short run, such as depreciation on a building or a long-term lease. These are structural and locked in.
Discretionary fixed cost
A fixed cost that management can adjust or eliminate relatively quickly, such as a new R&D programme, advertising, or training. The total does not change with volume, but the decision to incur it at all is revisited each budget cycle.
Semi-variable (mixed) cost
A cost that contains both a fixed component and a variable component. A salesperson's pay that includes a base salary plus commission per sale is the classic example.
Step cost
A cost that remains fixed over a range of activity but jumps to a new, higher level once activity crosses a threshold. On a graph, it looks like a staircase. Hiring an additional lab technician for every 150 patients is a step cost.
Product cost (inventoriable cost)
A cost that is attached to the product and flows through inventory accounts (Raw Materials, Work-in-Process, Finished Goods) before hitting Cost of Goods Sold on the income statement. Includes direct materials, direct labour, and manufacturing overhead.
Period cost
A cost that is expensed in the period it is incurred, rather than attached to a product. Selling expenses, administrative expenses, and corporate-level costs are period costs. They appear on the income statement directly, never in inventory.
Relevant range
The band of activity within which management expects the firm to operate and within which assumptions about cost behaviour (fixed costs stay flat, variable cost per unit stays constant) hold true. Outside this range, the patterns may break down.
Sunk cost
A cost that has already been incurred and cannot be recovered regardless of any future decision. Depreciation on equipment bought last year is a sunk cost. Sunk costs are never relevant to decision making.
Opportunity cost
The benefit foregone by choosing one alternative over the next best alternative. Opportunity costs are relevant to decisions even though they do not appear in the accounting records.
Contribution margin (per unit)
Selling price per unit minus all variable costs per unit. It represents the amount each unit contributes toward covering fixed costs and generating profit.
Contribution margin ratio (CMR)
Contribution margin per unit divided by selling price per unit. It tells you what percentage of each sales pound or dollar is left after variable costs to cover fixed costs and profit.
Every cost falls into one of four behaviour patterns when activity levels change:
Variable: Total changes proportionally with activity. Per-unit cost is constant. Examples: direct materials, direct labour (when paid per unit), variable manufacturing overhead, sales commissions.
Fixed: Total stays the same across the relevant range. Per-unit cost falls as volume rises. Examples: factory rent, insurance on the factory, depreciation on factory machinery, salaried managers.
Committed fixed: locked in by long-term decisions (building leases, equipment depreciation). Hard to cut in the short run.
Discretionary fixed: set each budget period and can be adjusted (R&D programmes, advertising, training). A new R&D programme is a discretionary fixed cost, not variable, because the total does not move with units produced.
Mixed (semi-variable): has both a fixed base and a variable component. Example: a salesperson paid a base salary plus commission. On a graph, the line starts above zero (the fixed portion) and slopes upward.
Step: fixed within a band, then jumps at a threshold. On a graph, it looks like a staircase. Example: one lab technician needed per 150 patients. Costs stay flat until patient 151, when a second technician is hired and the cost jumps.
This distinction determines where costs appear on the financial statements.
Product costs attach to inventory and only hit the income statement when the goods are sold (as Cost of Goods Sold). They include:
Direct materials (raw materials that become part of the finished product)
Direct labour (wages of workers who physically make the product)
Manufacturing overhead (all other factory costs: indirect materials, indirect labour, factory depreciation, factory insurance, factory utilities)
Period costs are expensed immediately in the period incurred. They include:
Selling expenses (sales commissions, advertising, shipping to customers)
Administrative expenses (executive salaries, office rent, corporate headquarters costs, CFO furniture depreciation, legal fees)
The critical test: if the cost is incurred on the factory floor to make the product, it is a product cost. If it is incurred to sell or administer the business, it is a period cost.
Tires installed on a new car by the manufacturer are a product cost (direct material). Sales commissions at a retail store are a period cost. Legal fees for a motel chain are a period cost. The CFO's furniture depreciation is a period cost.
The relevant range is the band of activity where management expects the firm to operate. Within it, the neat rules hold: total fixed costs stay flat, variable cost per unit stays constant. Move outside it, and the assumptions break. A factory might need a second shift (step increase in fixed costs) or secure volume discounts on materials (variable cost per unit drops).
When a problem says "within the relevant range," it is telling you to treat fixed costs as fixed and variable costs as proportional. If production is revised slightly downward, total fixed overhead stays the same, total variable overhead falls, and the variable cost per unit remains unchanged.
If production is revised downward:
Total fixed costs stay the same.
Fixed cost per unit increases (same total spread over fewer units).
Total variable costs decrease (fewer units, same rate per unit).
Variable cost per unit stays the same.
This is a favourite exam question. The answer to "what happens to total fixed overhead and the variable overhead rate when production drops" is: fixed overhead stays at the same total, and the variable rate per unit is unchanged (though total variable overhead falls).
The IMA (Institute of Management Accountants) sets four ethical standards:
Competence: maintain professional knowledge and follow laws.
Confidentiality: do not disclose confidential information.
Integrity: avoid conflicts of interest, refuse gifts that impair judgement.
Credibility: communicate information fairly and objectively.
"Loyalty" is not one of the four. This is a common exam trap.
Managerial accounting supports three day-to-day activities:
Planning: setting goals and outlining how to achieve them.
Directing operations: running the business day to day.
Controlling: measuring performance against plans and taking corrective action.
"Decision making" is embedded in all three rather than being a separate primary activity. "Measuring profitability" is a tool used within controlling, not a standalone management activity in the standard framework.
Y = A + B(X)
Y = total cost (the dependent variable)
A = total fixed costs (the intercept on a graph)
B = variable cost per unit of activity (the slope)
X = the number of units of activity (the independent variable, the cost driver)
Fixed cost per unit = A / X. This is not constant; it changes as X changes.
Variable cost per unit = B. This is constant within the relevant range.
Total cost per unit = Y / X = (A / X) + B.
A quick way to estimate the variable and fixed components of a mixed cost using only two data points: the highest and lowest activity levels observed.
Step 1: Variable cost per unit (B) = (Cost at highest activity - Cost at lowest activity) / (Highest activity - Lowest activity)
Step 2: Fixed cost (A) = Total cost at either point - B x Activity at that point
Step 3: Plug into Y = A + B(X) to estimate cost at any activity level.
Worked example from the exam: maintenance cost data with the highest activity at 400 hours ($4,000) and lowest at 150 hours ($2,000).
B = ($4,000 - $2,000) / (400 - 150) = $2,000 / 250 = $8.00 per hour
A = $4,000 - $8.00 x 400 = $4,000 - $3,200 = $800
At 320 hours: Y = $800 + $8.00 x 320 = $800 + $2,560 = $3,360
Note: February at 180 hours/$1,425 has the lowest cost but not the lowest activity. The high-low method uses the highest and lowest activity levels, not the highest and lowest costs. This is a common trap.
A statistical method that fits the best line through all the data points, not just the two extremes. The output gives:
Intercept = estimated fixed cost (A)
Slope = estimated variable cost per unit of activity (B)
R-squared = how well the model fits the data (0 to 1; closer to 1 is better)
To estimate cost: Y = Intercept + Slope x Activity level.
Worked example: Intercept = $2,150, Slope = $12.50, volume = 3,000 machine hours.
Y = $2,150 + $12.50 x 3,000 = $2,150 + $37,500 = $39,650
In the regression equation Y = 495,000 + 5.65X:
Y = total manufacturing overhead (the dependent variable)
495,000 = estimated fixed manufacturing overhead
5.65 = variable cost per machine hour
X = the number of machine hours (the independent variable, also known as the cost driver)
"X" is not the dependent variable. "X" is not total overhead. $495,000 is the fixed component, not the variable.
Understanding cost behaviour is how businesses set prices, plan capacity, and decide whether to accept special orders. A factory manager uses the fixed-variable split to know that adding a second shift will not double total costs (the building rent stays flat). A hospital administrator uses step-cost logic to know how many nurses to roster per patient band. Regression analysis is used by cost analysts to build predictive models for budgeting, stripping out noise that the high-low method misses by using only two data points.
Students often confuse the high-low method's selection criteria. You pick the highest and lowest activity levels, not the highest and lowest costs. If the month with the lowest cost is not the month with the lowest activity, you still use the lowest-activity month.
Students often think fixed cost per unit is constant. It is not. Total fixed cost is constant; per-unit fixed cost drops as volume rises. This trips people up in questions about what happens when production changes.
Students sometimes classify a new R&D programme as a variable cost because it is a new decision. It is a discretionary fixed cost: the decision to start it is discretionary, but the cost does not change with each unit produced.
"Loyalty" sounds like it should be an ethical standard for accountants. It is not in the IMA framework. The four are competence, confidentiality, integrity, and credibility.
⚠️ Know what happens to total fixed cost, per-unit fixed cost, total variable cost, and per-unit variable cost when volume changes. This is tested almost every sitting.
⚠️ Be able to classify any given cost as product or period. The tyre-on-car question (product cost) and CFO-furniture question (period cost) are classic formats.
⚠️ The high-low method will appear as a calculation question. Watch for the trap of using the lowest-cost data point instead of the lowest-activity data point.
⚠️ Regression output interpretation is tested conceptually: know what intercept, slope, and R-squared mean. You will not be asked to run a regression by hand.
⚠️ The cost equation Y = A + B(X): be able to identify what each letter represents. A/X = fixed cost per unit, B = variable cost per unit, Y = total cost, X = activity level.
⚠️ The staircase graph question (step costs) appears regularly. Know that it represents a step cost, not a mixed cost and not a pure fixed cost.
True or False: Total variable cost per unit stays the same as production volume increases.
True. Variable cost per unit is constant within the relevant range.
True or False: Fixed cost per unit decreases as production volume increases.
True. The same total fixed cost is spread over more units.
Fill in the blank: In the equation Y = A + B(X), the slope B represents the _______ cost per unit of activity.
Variable.
True or False: The high-low method uses the data points with the highest and lowest costs.
False. It uses the highest and lowest activity levels.
True or False: Depreciation on factory equipment is a period cost.
False. It is manufacturing overhead, which is a product cost.
Q: For decision making using relevant costs, depreciation resulting from prior purchases is what kind of cost, and is it relevant?
A: It is a sunk cost and therefore not relevant. The money has already been spent; no future decision can change it. (Answer: D)
Q: The cost of a new research and development programme would best be described as what type of cost?
A: Discretionary fixed. It is fixed because the total does not vary with production volume. It is discretionary because management chooses each period whether to undertake it. (Answer: C)
Q: ABC has budgeted variable overhead of $150,000 and fixed overhead of $240,000. If anticipated production is revised slightly downward, what happens?
A: Total fixed overhead stays at $240,000. The variable overhead rate per hour stays the same (the rate is constant per unit of activity). Total variable overhead would fall, but the per-hour rate does not change. (Answer: B)
Q: Which of the following is NOT a period cost: sales commissions at a retail store, a motel chain's legal fees, tyres installed on a car by the manufacturer, or the CFO's furniture depreciation?
A: Tyres installed on a car by the manufacturer. They are direct material, a product cost. The others are all selling or administrative expenses. (Answer: C)
Q: A staircase-shaped graph on a total cost vs activity chart represents which type of cost?
A: A step cost, such as hiring one lab technician per 150 patients. The cost stays flat within each band, then jumps at the threshold. (Answer: A)
Q: At 90,000 units, variable cost per unit is $8 and fixed cost per unit is $4. What is the total cost at 85,000 units?
A: Total fixed cost = $4 x 90,000 = $360,000 (fixed in total, so it stays at $360,000 regardless of volume). Total variable cost at 85,000 = $8 x 85,000 = $680,000. Total cost = $360,000 + $680,000 = $1,040,000. (Answer: B)
Q: Using the high-low method with the maintenance data provided, estimate the cost at 320 hours.
A: Highest activity = 400 hours, cost $4,000. Lowest activity = 150 hours, cost $2,000. Variable rate = ($4,000 - $2,000) / (400 - 150) = $8.00/hour. Fixed cost = $4,000 - ($8.00 x 400) = $800. At 320 hours: $800 + ($8.00 x 320) = $3,360.
Q: The regression equation for overhead is Y = 495,000 + 5.65X, where X is machine hours. Which statement is true?
A: "X represents the number of machine hours" is correct. X is the independent variable (the cost driver), not the dependent variable. $495,000 is the fixed overhead estimate, not the variable component. (Answer: D)
Q: Given Y = A + B(X), which expression represents fixed cost per unit?
A: A/X. Total fixed cost is A; dividing by the number of units X gives fixed cost per unit. (Answer: A)
Q: Which is NOT one of the ethical standards for managerial accountants: competence, confidentiality, integrity, credibility, or loyalty?
A: Loyalty. The IMA standards are competence, confidentiality, integrity, and credibility. (Answer: E)
Q: Which is NOT a primary day-to-day management activity: measuring profitability, directing operations, decision making, controlling, or planning?
A: Measuring profitability. The three primary activities are planning, directing operations, and controlling. (Answer: A)
Q: The relevant range is the range of activity where what?
A: Where management expects the firm to operate. It is not about maximum efficiency, profit, or units produced equalling units sold. (Answer: C)
This material feeds directly into CVP analysis: you cannot calculate a break-even point without first separating costs into fixed and variable. It also underpins relevant cost analysis for decisions like special orders and outsourcing, where you need to know which costs change with a decision and which do not. The cost estimation methods (high-low, regression) are used in budgeting and standard costing later in the course.
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