Optimization at the margin: comparing the extra benefit with the extra cost

Optimization. Economics assumes that people, firms and governments try to optimize: out of the options open to them, they choose the best one. Because resources are scarce, every choice has a cost, and rational decision makers weigh costs against benefits before they act.

Thinking at the margin. Most real choices are not all-or-nothing. They are about a little more or a little less: one more hour of study, one more unit of output, one more item bought. The margin is this edge, the small incremental change from where one now stands, and rational people decide by looking at it. Optimizing at the margin means asking, for each small step, how much extra do I gain, and how much extra does it cost, rather than looking at totals or averages.

The marginal rule.

• Take a step while its marginal benefit exceeds its marginal cost; stop where the two are equal. Beyond that point each further step adds more to cost than to benefit.
• For a firm, the extra revenue from one more unit is marginal revenue (MR) and the extra cost is marginal cost (MC). As long as MR is greater than MC, producing more raises profit; once MR falls below MC, more output lowers profit; so profit is highest where MR = MC, which for a price-taking firm means price = MC.
• For a consumer, each additional unit of a good usually adds less satisfaction than the one before, so a buyer takes one more unit only when its price has fallen far enough to match what that extra unit is worth to her.
• Costs that do not change with the decision are left out. A cost that has to be paid whatever is decided, such as a fixed rent, or a cost already incurred that cannot be recovered, does not belong at the margin.

Example (our own). A highway dhaba near Panipat normally closes at 10 pm and is deciding how long to stay open after that. Its monthly rent is paid either way, so it is ignored. For each extra hour, the owner estimates the extra takings and the extra cost of the cook's overtime, gas and electricity:

The first two hours each add more than they cost, so the owner stays open until midnight, adding ₹900 (700 + 200) to profit. The third hour would cost ₹200 more than it brings in, so he closes then. Looking at the rent per hour, or at the average takings of the whole day, would have given the wrong answer.

Conclusion. The principle of optimization at the margin says that a rational decision maker reaches the best choice step by step, expanding an activity while the extra benefit of one more unit exceeds its extra cost and stopping where the two are equal, and ignoring costs that the decision does not change.