The world economy can be evaluated in various ways, depending on the model used, and this valuation can then be represented in various ways (for example, in 2006 US dollars). It is inseparable from the geography and ecology of Earth, and is therefore somewhat of a misnomer, since, while definitions and representations of the "world economy" vary widely, they must at a minimum exclude any consideration of resources or value based outside of the Earth. For example, while attempts could be made to calculate the value of currently unexploited mining opportunities in unclaimed territory in Antarctica, the same opportunities on Mars would not be considered a part of the world economy – even if currently exploited in some way – and could be considered of latent value only in the same way as uncreated intellectual property, such as a previously unconceived invention. Beyond the minimum standard of concerning value in production, use, and exchange on the planet Earth, definitions, representations, models, and valuations of the world economy vary widely.
It is common to limit questions of the world economy exclusively to human economic activity, and the world economy is typically judged in monetary terms, even in cases in which there is no efficient market to help valuate certain goods or services, or in cases in which a lack of independent research or government cooperation makes establishing figures difficult. Typical examples are illegal drugs and prostitution, which by any standard are a part of the world economy, but for which there is by definition no legal market of any kind.
However, even in cases in which there is a clear and efficient market to establish a monetary value, economists do not typically use the current or official exchange rate to translate the monetary units of this market into a single unit for the world economy, since exchange rates typically do not closely reflect world-wide value, for example in cases where the volume or price of transactions is closely regulated by the government. Rather, market valuations in a local currency are typically translated to a single monetary unit using the idea of purchasing power. This is the method used below, which is used for estimating worldwide economic activity in terms of real US dollars. However, the world economy can be evaluated and expressed in many more ways. It is unclear, for example, how many of the world's 6.5 billion people have most of their economic activity reflected in these valuations.
