if he were to compare for you the lifestyle of a person with a Ph.D. to that of a grade school dropout, you might complain that this comparison is not helpful for your decision. you have some education already and most likely are deciding whether to spend an extra year or two in school. To make this decision, you need to know additional benefits that an extra year in school would offer and the additional costs that you would incur by comparing these marginal benefits and marginal costs,you can evaluate whether the extra year is worthwhile.