CEVAP:

Thinking at the margin requires recognizing the need to make decisions by evaluating the costs and benefits of marginal changes. It means that a decision maker should compare the marginal benefit and the marginal cost of his/her actions. Any action should be undertaken only if the benefit is greater than the cost. For example, the decision concerning whether to go to a university for an advanced degree or not must be made by comparing the costs (the fees and foregone wages) with the expected benefit (the extra income you could earn with the extra year of education).