The Goodmark Company example shows quite clearly that prime costs are assigned in
the same way for functional- or activity-based costing. The example also demonstrates
that the total amount of overhead costs is assigned under either approach. The amount
assigned to each product, though, can differ significantly, depending on which method
is used. The theoretical premise of activity-based costing is that it assigns costs according
to the resource consumption pattern of products. If this is true, then activity-based
costing should produce more accurate product costs if there is product diversity simply
because unit-based drivers cannot capture the full consumption pattern of products.
The Goodmark example suggests that we simply need to choose among a plantwide
cost pool, departmental cost pools, or activity cost pools. While this is true, it is also
true that we are talking about different levels of aggregation. In reality, if there is no
product diversity and a plantwide cost pool is chosen, all we need is the cost of overhead
resources taken from the general ledger accounts: depreciation, salaries, utilities,
rent, etc. On the other hand, departmental cost pools require more detail and less aggregation
because costs must be assigned to every producing department. Finally,
activity-based costing requires the most detail and the least aggregation because each
activity performed and its associated costs must be identified.