Exhibit 4-17 illustrates that the ABC costs are approximated quite well by the reduced
system of four drivers. For Wafer A, the error is about 2.5 percent [($820,000
$800,000)/$800,000], using the larger 12-driver system in Exhibit 4-16 as the
benchmark. If activity costs roughly follow the Pareto principle or 80/20 rule (80 percent
of the overhead costs are caused by 20 percent of the activities), then this approach
for reducing the size of the system has some promise. For example, if a system has 100
activities, then the top 20 activities (as measured by their cost) need to account for a
very high percentage of the total costs. In those cases where this holds, a reduced system
may work reasonably well because most of the costs are assigned using cause-andeffect
relationships. Even so, there may be some who would balk at the notion of using
15–20 drivers. The approach also loses its usefulness for those companies where a small
number of activities do not account for a large share of the overhead costs