Economists analyze relationships like revenue functions from the perspective of how the function changes in response to a small change in the quantity. These **marginal measurements** not only provide a numerical value to the responsiveness of the function to changes in the quantity but also can indicate whether the business would benefit from increasing or decreasing the planned production volume and in some cases can even help determine the optimal level of planned production.

The **marginal revenue** measures the change in revenue in response to a unit increase in production level or quantity. The **marginal cost** measures the change in cost corresponding to a unit increase in the production level. The **marginal profit** measures the change in profit resulting from a unit increase in the quantity. Marginal measures for economic functions are related to the operating volume and may change if assessed at a different operating volume level.

There are multiple computational techniques for actually calculating these marginal measures. If the relationships have been expressed in the form of algebraic equations, one approach is to evaluate the function at the quantity level of interest, evaluate the function if the quantity level is increased by one, and determine the change from the first value to the second.

Suppose we want to evaluate the marginal revenue for the revenue function derived in the previous section at last summer’s operating level of 36,000 ice cream bars. For a value of Q = 36,000, the revenue function returns a value of $54,000. For a value of Q = 36,001, the revenue function returns a value of $53,999.70. So, with this approach, the marginal revenue would be $53,999.70 − $54,000, or –$0.30. What does this tell us? First, it tells us that for a modest increase in production volume, if we adjust the price downward to compensate for the increase in quantity, the net change in revenue is a decrease of $0.30 for each additional unit of planned production.

Marginal measures often can be used to assess the change if quantity is decreased by changing sign on the marginal measure. Thus, if the marginal revenue is –$0.30 at Q = 36,000, we can estimate that for modest decreases in planned quantity level (and adjustment of the price upward based on the demand function), revenue will rise $0.30 per unit of decrease in Q.

At first glance, the fact that a higher production volume can result in lower revenue seems counterintuitive, if not flawed. After all, if you sell more and are still getting a positive price, how can more volume result in less revenue? What is happening in this illustrated instance is that the price drop, as a percentage of the price, exceeds the increase in quantity as a percentage of quantity. A glance back at Figure 2.5 confirms that Q = 36,000 is in the portion of the revenue function where the revenue function declines as quantity gets larger.

If you follow the same computational approach to calculate the marginal cost and marginal profit when Q = 36,000, you would find that the marginal cost is $0.30 and the marginal profit is –$0.60. Note that marginal profit is equal to marginal revenue minus marginal cost, which will always be the case.

The marginal cost of $0.30 is the same as the variable cost of acquiring and stocking an ice cream bar. This is not just a coincidence. If you have a cost function that takes the form of a linear equation, marginal cost will always equal the variable cost per unit.

The fact that marginal profit is negative at Q = 36,000 indicates we can expect to find a more profitable value by decreasing the quantity and increasing the price, but not by increasing the quantity and decreasing the price. The marginal profit value does not provide enough information to tell us how much to lower the planned quantity, but like a compass, it points us in the right direction.

Since marginal measures are the rate of change in the function value corresponding to a modest change in Q, differential calculus provides another computational technique for deriving marginal measures. Differential calculus finds *instantaneous* rates of change, so the values computed are based on infinitesimal changes in Q rather than whole units of Q and thus can yield slightly different values. However, a great strength of using differential calculus is that whenever you have an economic function in the form of an algebraic equation, you can use differential calculus to derive an entire function that can be used to calculate the marginal value at *any* value of Q.

How to apply differential calculus is beyond the scope of this text; however, here are the functions that can be derived from the revenue, cost, and profit functions of the previous section (i.e., those that assume a variable price related to quantity):

\[\text{marginal revenue at a volume }Q = $3.3 − $0.0001 Q,\]

\[\text{marginal cost at a volume }Q = $ 0.3,\]

\[\text{marginal profit at a volume }Q = $3 − $0.0001 Q.\]

Substituting \(Q = 36,000\) into these equations will produce the same values we found earlier. However, these marginal functions are capable of more.

Since the marginal change in the function is the rate of change in the function at a particular point, you can visualize this by looking at the graphs of the functions and drawing a tangent line on the graph at the quantity level of interest. A tangent line is a straight line that goes through the point on the graph, but does not cross the graph as it goes through the point. The slope of the tangent line is the marginal value of the function at that point. When the slope is upward (the tangent line rises as it goes to the right), the marginal measure will be positive. When the slope is downward, the marginal measure will be negative. If the line has a steep slope, the magnitude of the marginal measure will be large. When the line is fairly flat, the magnitude will be small.

Suppose we want to find where the profit function is at its highest value. If you look at that point (in the vicinity of Q = 30,000) on Figure 2.5, you see it is like being on the top of a hill. If you draw the tangent line, it will not be sloped upward or downward; it will be a flat line with a zero slope. This means the marginal profit at the quantity with the highest profit has a value of zero. So if you set the marginal profit function equal to zero and solve for Q you find

\[0 = $3.00 − $0.0001 Q\]

implies

\[Q = \dfrac{$3.00}{$0.0001} = 30,000.\]

This confirms our visual location of the optimum level and provides a precise value.

This example illustrates a general economic principle: Unless there is a constraint preventing a change to a more profitable production level, the **most profitable production level** will be at a level where marginal profit equals zero. Equivalently, in the absence of production level constraints, the most profitable production level is where marginal revenue is equal to marginal cost. If marginal revenue is greater than marginal cost at some production level and the level can be increased, profit will increase by doing so. If marginal cost is greater than marginal revenue and the production level can be decreased, again the profit can be increased.