Short-Term Decision-Making

Question: We know that some costs vary with changes in activity. What do we call this type of cost behavior?

Answer: This cost behavior pattern is called a variable cost. A variable cost describes a cost that varies in total with changes in volume of activity. The activity in this example is the number of bikes produced and sold. However, the activity can take many different forms depending on the organization. The two most common variable costs are direct materials and direct labor. Other examples include indirect materials and energy costs.

Assume the cost of direct materials (wheels, seats, frames, and so forth) for each bike at Bikes Unlimited is $40. If Bikes Unlimited produces one bike, total variable cost for direct materials amounts to $40. If Bikes Unlimited doubles its production to two bikes, total variable cost for direct materials also doubles to $80. Variable costs typically change in proportion to changes in volume of activity. If volume of activity doubles, total variable costs also double, while the cost per unit remains the same. It is important to note that the term variable refers to what happens to total costs with changes in activity, not to the cost per unit.

Taking it one step further for Bikes Unlimited, let's consider all variable costs related to production. Assume direct materials, direct labor, and all other variable production costs amount to $60 per unit. Table 5.1 "Variable Cost Behavior for Bikes Unlimited" provides the total and per unit variable costs at three different levels of production, and Figure 5.1 "Total Variable Production Costs for Bikes Unlimited" graphs the relation of total variable costs (y-axis) to units produced (x-axis). Note that the slope of the line represents the variable cost per unit of $60 (slope = change in variable cost ÷ change in units produced).

Table 5.1 Variable Cost Behavior for Bikes Unlimited

Figure 5.1 Total Variable Production Costs for Bikes Unlimited

Using Different Activities to Measure Variable Costs

Question: At Bikes Unlimited, it is reasonable to assume that the activity, number of units produced, will affect total variable costs for direct materials and direct labor. However, companies often use a different activity to estimate total variable costs.What types of activities might be used to estimate variable costs?

Answer: The type of activity used to estimate variable costs depends on the cost. For example, a law firm might use the number of labor hours to estimate labor costs. An airline such as American Airlines might use hours of flying time to estimate fuel costs. A mail delivery service such as UPS might use the number of packages processed to estimate labor costs associated with sorting packages. A retail store such as Best Buy might use sales dollars to estimate cost of goods sold.

Variable costs are affected by different activities depending on the organization. The goal is to find the activity that causes the variable cost so that accurate cost estimates can be made.