What it means
Every cost has a cause. Direct costs have obvious ones: materials cost is caused by the units made, and the link is visible.
Overheads have less obvious causes: the cost of the scheduling department is caused by the number of production runs it schedules, not by the number of units produced; the cost of the purchasing department by the number of orders and suppliers, not by the value purchased; the cost of customer service by the number and complexity of enquiries, not by revenue. A cost driver names the cause, and once named, the cost can be assigned to whatever generated the cause and managed by managing the cause.
The idea's main application is activity-based costing. Traditional costing assigns overhead to products by a volume measure such as labour hours, which assumes that overhead is caused by volume.
In modern operations much of it is not: it is caused by transactions (each set-up, order, inspection, delivery, engineering change) and by complexity (each additional product, customer, supplier, site). A product made in small batches with frequent set-ups, special handling and its own engineering attention causes far more overhead per unit than a high-volume standard product, and volume-based allocation hides it.
Activity-based costing identifies the activities, finds the driver of each, calculates a cost per unit of driver, and charges each product for the driver units it consumes. The result is a product cost that reflects what the product actually causes the business to do.
Drivers are classified by level. Unit-level drivers vary with each unit produced (machine hours, materials).
Batch-level drivers vary with each batch (set-ups, material movements, purchase orders). Product-level drivers vary with each product line maintained (engineering support, product specifications, marketing per product).
Customer-level drivers vary with each customer (sales visits, special terms, invoicing). Facility-level costs (the plant, the head office) have no product driver and are allocated arbitrarily or not at all.
The hierarchy matters because a decision that changes volume affects only unit-level costs, while a decision to add a product or a customer affects the higher levels. Choosing a driver requires a causal link, measurability and practicality.
The number of set-ups is a good driver for set-up cost if set-ups are similar; if they vary widely in duration, set-up hours is better. The number of purchase orders drives purchasing cost, but if orders vary in complexity, order lines may be closer.
Perfect drivers do not exist, and the aim is a driver close enough to the cause that the assigned costs are decision-useful, measured at a cost less than the value of the information. In cost management, the driver analysis points to two levers.
Reduce the driver: fewer set-ups through larger batches or faster changeovers, fewer orders through consolidation, fewer products through range rationalisation, fewer suppliers. Reduce the cost per driver unit: automate order processing, standardise set-ups, simplify the product structure so that each engineering change costs less.
Programmes that cut cost without changing the drivers tend to see the cost return; programmes that attack drivers change the cost structure. Structural drivers operate above the level of activities: the number of plants, the breadth of the product range, the number of markets, the degree of vertical integration, the technology employed.
These are set by strategy, and they determine the level of overhead the business will carry regardless of how efficiently each activity is run.
In practice
Real-world examples.
Example
A hospital identifies bed-days, theatre minutes and diagnostic tests as the drivers of its clinical costs, and finds that the cost of a procedure varies more with length of stay than with the operation itself.
Example
A software company finds that the number of customer-specific configurations drives 40% of its support cost and limits configurability in its next release.
Example
A bank identifies the number of accounts, not their balances, as the driver of its operations cost and closes 200,000 dormant accounts.
Think of it
“A cost driver is what causes a cost to happen-the factor that drives the expense.
Formula
Calculation
Cost Driver Rate = Activity cost pool / Total driver units
Cost assigned to a product = Sum over activities of (Driver rate x Driver units consumed by the product)
Cost reduction through the driver = (Driver units before minus Driver units after) x Driver rate
Cost reduction through the rate = Driver units x (Rate before minus Rate after)
Worked example. A distributor's warehouse overhead is $2,400,000 a year, previously allocated to customers as a percentage of revenue (revenue $40,000,000: 6%). Activity analysis identifies four activities and their drivers:
- Order picking: $960,000; driver: order lines picked; 480,000 lines a year; rate $2.00 per line
- Receiving and put-away: $480,000; driver: pallets received; 24,000 pallets; rate $20.00 per pallet
- Order processing and dispatch: $600,000; driver: orders shipped; 60,000 orders; rate $10.00 per order
- Returns handling: $360,000; driver: returns processed; 9,000 returns; rate $40.00 per return
Two customers, each with revenue of $2,000,000 (5% of the total), previously allocated $120,000 of warehouse overhead each.
Customer A, a supermarket chain: 8,000 orders a year (many small replenishment orders), 60,000 lines, 1,500 returns. Receiving is customer-independent and allocated by revenue share: 5% of $480,000 = $24,000.
- Picking 60,000 x $2.00 = $120,000; processing 8,000 x $10.00 = $80,000; returns 1,500 x $40.00 = $60,000; receiving $24,000. Total $284,000, or 14.2% of revenue.
Customer B, an industrial buyer: 400 orders a year (large monthly orders), 20,000 lines, 100 returns.
- Picking $40,000; processing $4,000; returns $4,000; receiving $24,000. Total $72,000, or 3.6% of revenue.
Same revenue, and Customer A costs four times as much to serve. At a 20% gross margin ($400,000 each), Customer A's contribution after warehouse cost is $116,000 and Customer B's $328,000. Under the revenue-based allocation both had shown $280,000. The distributor's sales team had been offering Customer A the same terms as Customer B.
Actions through the drivers:
- Customer A's orders: negotiate a minimum order size and a weekly consolidated order, cutting orders from 8,000 to 2,000 and lines from 60,000 to 45,000 (larger orders, same products): saving $60,000 of processing and $30,000 of picking
- Customer A's returns: analysis shows 60% of returns arise from a mismatch between the customer's order codes and the distributor's; a code alignment cuts returns from 1,500 to 600: saving $36,000
- Picking rate: a warehouse reorganisation cuts picking cost per line from $2.00 to $1.70 across all customers: saving $144,000 across the business
- Customer A's terms: a 1.5% price adjustment for orders below the minimum, which recovers the cost of the remaining small orders or removes them
Customer A's warehouse cost falls to about $150,000 (7.5% of revenue) and its contribution after warehouse cost rises to $250,000. The distributor introduces a customer profitability report built on the driver-based costs, and its sales incentive plan moves from revenue to contribution after cost to serve.
Structural driver: the distributor carries 12,000 product lines, of which 4,000 sell fewer than 10 units a year. Each line drives receiving, storage, catalogue and system costs estimated at $180 a year regardless of volume: $720,000 for the slow lines against $190,000 of gross margin from them. The range is cut to 8,500 lines, and the structural cost falls by about $500,000 after allowing for lines retained for strategic customers.Case study
Seen in the real world.
An engineering company's overhead had grown from 35% to 52% of cost over a decade while its volume was flat, and successive cost-cutting programmes had trimmed headcount in each department only to see the numbers grow back. A driver analysis found the cause: the company's product range had grown from 300 to 2,200 variants, its supplier base from 400 to 1,400, and its average batch size had fallen by 60%, so that the number of set-ups, purchase orders, engineering changes, inspections and schedule revisions had tripled. Every overhead department had been staffed to handle the transactions, and every cut had been reversed by the transaction volume.
The company attacked the drivers rather than the departments: it cut the range to 900 variants (analysis showed 1,300 contributed 4% of revenue), consolidated suppliers to 500, introduced a minimum batch size, and charged internal engineering changes to the product managers who requested them. Transaction volumes fell by half over eighteen months; overhead departments shrank through attrition to two thirds of their size; overhead fell to 40% of cost and stayed there. The finance director's observation was that the company had spent a decade cutting the people who did the work and never asked what was generating it.
Watch out
Common mistakes.
- Allocating overhead on a volume base (units, hours, revenue) when the cost is caused by transactions or complexity, which hides the cost of small, complex products and demanding customers.
- Cutting overhead departments without reducing the drivers that generate their work, which produces temporary savings that reverse.
- Choosing drivers for ease of measurement rather than causal link, which reintroduces the misallocation activity-based costing was meant to fix.
Questions
People also ask.
What is the difference between a cost driver and an allocation base?
An allocation base is any measure used to spread costs; a cost driver is an allocation base with a causal relationship to the cost. Labour hours are an allocation base for factory overhead; set-ups are a cost driver for set-up cost.
How many cost drivers does a business need?
Enough to capture the main causes of its significant overheads, typically five to fifteen activities with one driver each. More precision costs more to maintain than it is worth.
What is a structural cost driver?
A characteristic of the business's design that determines its cost level: product range, number of sites, degree of integration, technology. These are set by strategy and changed by strategic decisions, not by operational efficiency.
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