Agenda-setting theory describes the "ability [of the news media] to influence the salience of topics on the public agenda." That is, if a news item is covered frequently and prominently the audience will regard the issue as more important. Agenda-setting theory was formally developed by Dr. Max McCombs and Dr. Donald Shaw in a study on the 1968 presidential election. In the 1968 "Chapel Hill study," McCombs and Shaw demonstrated a strong correlation between what 100 residents of Chapel Hill, North Carolina thought was the most important election issue and what the local and national news media reported was the most important issue. By comparing the salience of issues in news content with the public's perceptions of the most important election issue, McCombs and Shaw were able to determine the degree to which the media determines public opinion. Since the 1968 study, published in a 1972 edition of Public Opinion Quarterly, more than 400 studies have been published on the agenda-setting function of the mass media, and the theory continues to be regarded as relevant.
Agenda setting occurs through a cognitive process known as "accessibility." Accessibility implies that the more frequently and prominently the news media cover an issue, the more instances of that issue become accessible in audience's memories. When respondents are asked what the most important problem facing the country is, they answer with the most accessible news issue in memory, which is typically the issue the news media focus on the most. The agenda-setting effect is not the result of receiving one or a few messages but is due to the aggregate impact of a very large number of messages, each of which has a different content but all of which deal with the same general issue. Mass-media coverage in general and agenda-setting in particular also has a powerful impact on what individuals think that other people are thinking, and hence they tend to allocate more importance to issues that have been extensively covered by mass media.
The agenda-building perspective ascribes importance not only to mass media and policymakers, but also to social process, to mutually interdependent relation between the concerns generated in social environment and the vitality of governmental process. Several real-world examples provide evidence that the Internet-community, particularly bloggers, can push their own agenda into public agenda, then media agenda, and, eventually, into policy agenda. For instance, in 2005 Eason Jordan, the chief news executive at CNN, abruptly resigned after being besieged by the online community after saying, according to various witnesses, that he believed the United States military had aimed at journalists in Iraq and killed 12 of them. Similarly, in 2002 Senate majority leader Trent Lott had to resign due to his inappropriate racist remarks that were widely discussed in the blogosphere. However bloggers attract attention not only to oust journalists and politicians. An online investigation on technical problems with electronic voting machines started by an activist Bev Harris in 2003 eventually forced traditional media outlets to address issue of electronic voting malperformance. This in turn made Diebold, a company that produces these machines, to acknowledge its fault and take measures to fix it.
As a result in the changes in technology, there have been major changes in the ways in which people receive their news. Newspapers, broadcast television, and terrestrial radio are all examples of "vertical media" which is rapidly declining. Now the more common form of media is "horizontal media." The main differences are that it is more specialized and people pay premiums for this type of media. Horizontal media includes cable television and satellite radio as well as other media that is paid for. Horizontal and vertical media intersect in virtual brand communities, or the Internet. This is because the Internet is free like vertical media but serves specialized interest groups like horizontal media. Now people seek news in different ways, the media and its agenda have had to adapt. Although the major tenets of agenda setting theory have maintained their importance with the changes of new media, an aspect of agenda setting theory has changed. This change is known as Agenda Melding which focuses "on the personal agendas of individuals vis-à-vis their community and group affiliations". This means that individuals join groups and blend their agendas with the agendas of the group. Then groups and communities represent a "collected agenda of issues" and "one joins a group by adopting an agenda." On the other hand, agenda setting defines groups as "collections of people based on some shared values, attitudes, or opinions" that individuals join. This is different from traditional agenda setting because according to Shaw et al. individuals join groups in order to avoid social dissonance and isolation that is also known as "need for orientation". Therefore in the past in order to belong people would learn and adopt the agenda of the group. Now with the ease of access to media, people form their own agendas and then find groups that have similar agendas that they agree with. The advances in technology have made agenda melding easy for people to develop because there is a wide range of groups and individual agendas. The Internet makes it possible for people all around the globe to find others with similar agendas and collaborate with them. In the past agenda setting was limited to general topics and it was geographically bound because travel was limited.
