8.2 How costs behave
Different kinds of costs are said to 'behave' in different ways. Fixed costs are, as the name suggests, fixed and will remain at the same level regardless of the number of units of output. For example, rent of a building doesn't change with a change in the number of units produced, but variable costs relate directly to the units of output and are expressed as dollars per unit. Variable costs, such as the cost of raw materials, increase as the number of units of output increases and vice versa.
While there is a tendency for fixed costs to be indirect and variable costs to be direct this is not always true. The concepts of direct/indirect and fixed/variable are very different and many operations have variable overheads (an indirect cost) or direct labour as a fixed cost, particularly over a short period of time.
If we add together the fixed costs and the variable costs we get full cost. Also if we add together the direct costs and the indirect costs we will also get full cost. We arrive at the same destination using different paths. With a single product/service organisation we can simply add the fixed and variable costs and allocate the total across all units of output, but with a multi product/service organisation we must allocate the direct costs to the units of output and apportion the indirect costs 'fairly' across the units of output.
A major issue and a difficult practical problem is how to deal with overhead costs.