Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Opportunity cost is the value of the next-best choice available to someone who has picked between several mutually exclusive choices. It is a key concept in economics. It is a calculating factor used in mixed markets which favour social change in favour of purely individualistic economics. It has been described as expressing "the basic relationship between scarcity and choice." The notion of opportunity cost plays a crucial part in ensuring that scarce resources are used efficiently. Thus, opportunity costs are not restricted to monetary or financial costs: the real cost of output forgone, lost time, swag, pleasure or any other benefit that provides utility should also be considered opportunity costs. The concept of an opportunity cost was first developed by John Stuart Mill.