Let's illustrate this by looking at the cost of property insurance. The cost of insuring the factory building is a fixed cost when the independent variable is the number of units produced within the factory. In other words, the factory's property insurance might be $6,000 per year whether its output is 2 million units, 3 million units, or 5 million units. On the other hand, if the independent variable is the replacement cost of the factory buildings, the insurance cost will be a variable cost. The reason is the insurance cost on $12 million of factory buildings will be more than the insurance cost on $9 million of factory buildings, and less than the insurance premiums on $18 million of factory buildings.
In the case of worker compensation insurance, the cost will vary with the amount of payroll dollars (excluding overtime premium) in each class of workers. For example, if the worker comp premiums are $5 per $100 of factory labor cost, then the worker comp premiums will be variable with respect to the dollars of factory labor cost. If the units of output in the factory correlate with the direct labor costs, then the worker compensation cost will also be variable with respect to the number of units produced. On the other hand, the worker compensation cost for the office staff is usually a much smaller rate and that worker compensation cost will not be variable with respect to the number of units of output in the factory. However, the worker compensation cost of the office staff will be variable with respect to the amount of office staff salaries and wages.
As you have seen, determining which costs are fixed and which are variable can be a bit tricky.
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