A cost driver is the activity whose level causes a cost to change, and the term carries two senses that sit side by side. In cost behavior it names the activity a cost moves with, since a cost driver is defined as any activity that causes the organization to incur a variable cost, with direct labor hours, machine hours, units produced and units sold given as the examples (OpenStax, Principles of Accounting, Volume 2: Managerial Accounting, section 2.2). In cost assignment it names the activity measure a pool of indirect cost is charged out on, because activity-based costing systems use cost drivers to assign costs through activities to outputs (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 148).
One test runs through both senses, and it is causation rather than correlation. The common overhead bases count as cost drivers because of the causal relationship between the base and the related costs, so that as the cost driver’s usage increases, the cost of overhead increases as well (OpenStax, Principles of Accounting, Volume 2: Managerial Accounting, section 6.2). A measure that merely moves alongside a cost has not earned the name.
What Counts as a Cost Driver
An activity does, and an activity is any action that consumes company resources, such as taking orders for a product, setting up machines to produce the product, inspecting the product, and providing customer support before and through the order process (OpenStax, section 6.2).
The level the activity sits at matters as much as the activity itself. Some overhead costs, such as inspection, increase with each unit inspected and have to be allocated at a per-unit level, while others, such as machine setup costs, are incurred each time a batch of products is manufactured and have to be allocated at the batch level (OpenStax, section 6.2). The federal managerial cost accounting standard uses the same ladder, classifying activities into unit-level, batch-level, product sustaining, and facility sustaining activities (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 149).
That ladder is what separates the two families of driver. Volume-based drivers measure how much the operation ran, and the most common bases for predetermined overhead are direct labor hours, direct labor dollars, or machine hours (OpenStax, section 6.2). Activity-based drivers count events instead, and a pool built out of machine setups follows the number of setups however few units each run produces, while a pool built out of quality inspections follows the inspections and not the labor hours around them (OpenStax, section 6.2). The same source lists the number of product returns from customers, the number of square feet, the number of customer contacts, the number of employees and the number of customer orders among the measures worth testing, which is the point: nothing about a driver has to be measured in hours.
Transaction Drivers and Duration Drivers
Two drivers can name the same activity and still count different things. A driver may be the number of times an activity is performed in producing a specific type of output, which the federal standard calls the transaction driver, or the length of time an activity is performed, which it calls the duration driver (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 149).
Counting occurrences is the cheaper measure and it carries an assumption: that every occurrence of the activity takes roughly the same effort. Where that holds, the count is enough. Where one occurrence routinely runs several times longer than another, the duration measure is the one that tracks the cost, and it has to be collected occurrence by occurrence to be worth anything.
What a Cost Driver Is Confused With
A Cost Object
The cost object is the thing being costed. The cost driver is the measure that moves what it costs. The term cost object refers to an activity or item whose cost is to be measured, and in a broad sense a cost object can be an organizational division, program, activity, task, product, service, or customer (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 121). So a client is a cost object and the transactions that client generates are a driver, and the two never swap places in the same sentence.
An Allocation Base
Every driver used in assignment is an allocation base, and not every allocation base is a driver. For cost allocation purposes, indirect costs may be grouped into pools, each pool is subject to one allocation base, and the base should be used consistently to allow cost comparison from one period to another (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 136).
The base only earns the name driver when the causal link is really there, and the same standard is blunt about how often it is not. Users of the cost information should be aware that distortions in product costing often result from arbitrary cost allocations, since in most cases there is little correlation between an indirect cost and the allocation base, and the allocation is arbitrary (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 137). That is why the standard ranks the methods rather than treating them as equal, putting directly tracing costs wherever economically feasible first, assigning costs on a cause-and-effect basis second, and allocating costs on a reasonable and consistent basis last (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 124). A driver is what lifts a base out of the third tier and into the second. Choosing the base and building a rate on it belongs to costing overheads and to predetermined overhead, and the act of assignment itself is allocation.
A Cost Pool
The pool is the bucket of cost. The driver is the measure it is charged out on. What ties them is that costs with a similar cause-and-effect relationship to outputs can be grouped into cost pools, a similarity referred to in some literature as the cost pool homogeneity concept (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 129). The driver therefore decides pool membership as well as pool rate, because outputs require the performance of certain activities and the activities cause costs, so an activity is considered a linkage between the cause and the effect (FASAB, Statement of Federal Financial Accounting Standards 4, paragraph 130). Splitting one pool into several so that each carries its own driver and its own rate is activity based costing.
The Cost Itself
A driver is a measure, not money. Naming a driver says what to count, and it never says what was spent, so a driver can be named for a pool nobody has totalled yet. How a variable cost’s total then moves with the level of its driver, and what makes a cost variable in the first place, are worked through in variable expenses and how to determine variable cost. Treating one driver as the only thing that moves an entire business is a modelling assumption rather than a finding, and cost-volume-profit analysis sets out what that assumption costs.
Cost Drivers Inside a US CPA Firm
A firm’s overhead does not behave on billable hours just because billable hours are what the firm sells. Which activity a firm’s variable costs move with is named in variable expenses; the question here is narrower, which pool each candidate is honest as a base for.
Returns filed suits whatever scales with filing volume, such as e-file charges and per-return software cost. Review hours suits the pool made of senior time, which does not follow return counts at all, because one complicated return absorbs review that several simple ones never touch. Client onboardings suits the setup pool, since collecting records, opening the file and granting system access happens once per client rather than once per return. Transactions processed suits the bookkeeping pool, and it is the measure that separates a modest-fee client with heavy volume from a larger-fee client with almost none.
Picking any of them has a running cost. The driver has to be counted for every cost object, period after period, so the finer split earns its keep only where the finer answer would change a decision. Which internal reports all of this feeds belongs to managerial accounting, and the wider family of methods sits in cost accounting.
Common Questions
The activity whose level causes a cost to change. It is used two ways that rest on the same causal claim: the activity a cost moves with, and the measure a pool of indirect cost is charged out on.
Machine hours is the volume-based example, since the most common bases for predetermined overhead are direct labor hours, direct labor dollars, or machine hours (OpenStax, section 6.2). Machine setups and quality inspections are the activity-based ones, because setup cost is incurred per batch and inspection cost per unit inspected, so neither is visible in a labor-hour count (OpenStax, section 6.2).
They belong to managerial and cost accounting rather than to the face of the financial statements. Choosing a driver changes which cost object carries which share of a pool, and it leaves the pool’s own total exactly where it was. Whether a cost can be tied to one object without any driver at all is the separate traceability question in direct cost and overhead.
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