Maybe you made a big decision, such as investing in a mutual fund. Or a simple one such as buying a coffee maker or choosing a restaurant for dinner. Regardless of the decision, the decision process often includes evaluating the costs and benefits of each choice. For decisions that involve costs, some costs are irrelevant. For example, once you purchase a coffee maker, its cost is irrelevant when calculating how much money you save each time you brew coffee at home versus buy it at Starbucks. You incurred the cost of the coffee maker in the past, and you can’t recoup that cost. This chapter will explain which costs and benefits are relevant and which are not—and how you should think of them when choosing among alternatives.