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 other black market goods, which by any standard are a part of the world economy, but for which there is by definition no legal market of any kind.