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The public interest refers to the "common well-being" or "general welfare". The public interest is central to policy debates, politics, democracy and the nature of government itself. While nearly everyone claims that aiding the common well-being or general welfare is positive, there is little, if any, consensus on what exactly constitutes the public interest, or whether the concept itself is a coherent one.
According to the Random House Dictionary, Public interest is "1. the welfare or well-being of the general public; commonwealth. 2. appeal or relevance to the general populace: a news story of public interest."
Economist Lok Sang Ho in his Public Policy and the Public Interest (Routledge, 2012, published 2011) argues that the public interest must be assessed impartially and, therefore, defines the public interest as the "ex ante welfare of the representative individual."  Under a thought experiment, by assuming that there is an equal chance for one to be anyone in society and, thus, could benefit or suffer from a change, the public interest is by definition enhanced whenever that change is preferred to the status quo ex ante. This approach is "ex ante", in the sense that the change is not evaluated after the fact but assessed before the fact without knowing whether one would actually benefit or suffer from it.
This approach follows the "veil of ignorance" approach, which was first proposed by John Harsanyi but popularized by John Rawls in his 1971 ''Theory of Justice. Historically, however, the approach can be traced to John Stuart Mill, who, in his letter to George Grote, explained that "human happiness, even one's own, is in general more successfully pursued by acting on general rules, than by measuring the consequences of each act; and this is still more the case with the general happiness, since any other plan would not only leave everybody uncertain what to expect, but would involve perpetual quarrelling..."(in Francis E. Mineka and Dwight N. Lindley (ed.), The Collected Works of John Stuart Mill, Volume XIV - The Later Letters of John Stuart Mill 1849-1873 Part I]], Toronto: University of Toronto Press, London: Routledge and Kegan Paul, 1972, Vol. XV, p. 762, 1862).
Problems with the ex post or consequential approach 
The definitions of public interest based on the ex post or consequential approach are unavoidably debatable, leading to ambiguity and confusion.
Based on the ex post approach, for instance, there are different views on how many members of the public must benefit from an action before it can be declared to be in the public interest: at one extreme, an action has to benefit every single member of society in order to be truly in the public interest; at the other extreme, any action can be in the public interest as long as it benefits some of the population and harms none. But these extreme views are clearly not very useful in practice, since most cases of public policy involve some people gaining and some people losing.
Nicholas Kaldor and John Hicks offer two alternative but related ways to resolve the problem. The basic concept is that the gainers must gain more than the losers lose. Kaldor stated that the gainers must be able to compensate all the losers and still go along with the change, if the change is in the public interest. Hicks stated that the losers must NOT be able to bribe the gainers from forgoing the change, if the change is good for the public interest. It is observed that the Kaldorian position, if the compensations actually take place, is no different from the Pareto improvement criterion for enhancing social welfare. But if compensations do not actually take place, with gainers merely "potentially compensating the losers," people will not come to a consensus and agree that the change enhances the public interest.
But it should be clear that some acts in the public interest can be bad for some individuals. There may be an agreement that some interests are unique to the public. Stephen Krasner, a political scientist used a similar methodology in his book Defending the National Interest. Krasner identifies cases in which no corporate interest is found in US foreign policy in order to identify and analyze a national interest. The ex ante approach to the definition of the public interest would encompass this.
Public interest law 
According to Edwin Rekosh, Executive Director of PILnet: The Global Network for Public Interest Law (www.pilnet.org), "public interest law does not describe a body of law or a legal field; the term was adopted to describe whom the public interest lawyers were representing, rather than what matters they would work on. Instead of representing powerful economic interests, they chose to be advocates for those living in poverty. The term has grown, however, to encompass a broader range of activities of lawyers and non-lawyers working toward civil rights, civil liberties, women's rights, consumer rights, environmental protection, and 'fighting for the little guy'; - that is, representing vulnerable segments of society." 
Public interest law is widely understood as including at least civil representation through legal aid organizations and indigent criminal defense provided either by legal aid or the public defender (depending on the country). Other non-profit organizations that provide legal advice and representation to poor people also fall under the umbrella of public interest law.
United Kingdom public interest law 
In law, public interest is a defence against certain lawsuits (for instance some libel suits in the United Kingdom) and an exemption from certain laws or regulations (for instance freedom of information laws in the UK).
Public interest & the government 
Public interest has been considered as the core of "democratic theories of government” and often paired with two other concepts, "convenience" and "necessity." Public interest, convenience and necessity appear first time in the Transportation Act of 1920 and also appear in the Radio Act of 1927. After that, these three concepts became critical criteria for making communication policies and solving some related disputes.
Public interest & communication policies 
Public interest is at the heart of many communication laws. To serve the public interests has been one of the main goals of communication policies.
According to the Communications Act of 1934, SEC. 303. "Except as otherwise provided in this Act, the Commission from time to time, as public convenience, interest, or necessity requires, shall-...(g) Study new uses for radio, provide for experimental uses of frequencies, and generally encourage the larger and more effective use of radio in the public interest."
The Telecommunications Act of 1996 requires the regulators must make sure all regulation is consistent with the requirement of "public interest, convenience, and necessity." (e.g. SEC. 251. INTERCONNECTION (2)(B))
See also 
- Common good
- Condorcet paradox
- National interest
- Pareto optimality
- Radio Act of 1927
- Telecommunications Act of 1996
- Ho, Lok Sang, Public Policy and the Public Interest, Routledge, 2011
- Rawls, John (1971) A Theory of Justice, Cambridge: Harvard University Press.
- For information on the changing nature of public interest law see Paula O'Brien, 'Changing Public Interest Law: Overcoming the law's barriers to social change lawyering' (2011) 32 AltLJ 80. 
- Napoli, Philip M. (2001). Foundations of Communications Policy. Principles and Process in the Regulation of Electronic Media. Cresskill, NJ: Hampton Press