This means that a viewing of the movie is excludable and non-paying consumers are unable to experience the movie. Semi-Excludable Ranging between being fully excludable and non-excludable is a continuous scale of excludability that Ostrom developed. Within this scale are goods that either attempt to be excludable but cannot effective or efficiently enforce this excludability. One example concerns many forms of information such as music, movies, e-books and computer software. All of these goods have some price or payment involved in their consumption, but are also susceptible to piracy and copyright infringements. This can result in many non-paying consumers being able to experience and benefit from the goods of a single purchase or payment. Non-Excludable A good, service or resource that is unable to prevent or exclude non-paying consumers from experiencing or using it can be considered non-excludable. An architecturally pleasing building, such as Tower Bridge, creates an aesthetic non-excludable good, which can be enjoyed by anyone who happens to look at it. It is difficult to prevent people from gaining this benefit. A lighthouse acts as a navigation aid to ships at sea in a manner that is non-excludable since any ship out at sea can benefit from it. Implications and inefficiency Public goods will generally be underproduced and undersupplied in the absence of government subsidies, relative to a socially optimal level. This is because potential producers will not be able to realize a profit (since the good can be obtained for free) sufficient to justify the costs of production. In this way the provision of non-excludable goods is a classic example of a positive externality which leads to inefficiency. In extreme cases this can result in the good not being produced at all, or it being necessary for the government to organize its production and distribution. A classic example of the inefficiency caused by non-excludability is the tragedy of the commons (which Hardin, the author, later corrected to the 'tragedy of the unmanaged commons' because it is based on the notion of an entirely rule-less resource) where a shared, non-excludable, resource becomes subject to over-use and over-consumption, which destroys the resource in the process. Economic theory Brito and Oakland (1980) study the private, profit-maximizing provision of excludable public goods in a formal economic model. They take into account that the agents have private information about their valuations of the public good. Yet, Brito and Oakland only consider posted-price