Because decisions on what media outlets will cover are ultimately made by people, some degree of bias is inevitable. In the process of choosing what news to cover, editors and producers engage in agenda setting, the process by which the media determines what issues people think about and are concerned about. The idea of agenda setting rests on two basic assumptions:
- The media filters and shapes what we see.
- The more attention the media gives to a topic, the more likely the public will find it important.
Through agenda-setting, the media has some ability to control, or at least shape, what citizens care about.
Another source of bias is framing, which is sometimes difficult to observe. Framing is the media practice of focusing on certain details of a news story to tell it in a specific manner to the news audience. While framing makes the news understandable, it can also have a significant impact on how an audience views a topic. The condensing of complex stories can influence the attitudes that the audience develops. During election campaigns, media companies often come under criticism for what is called "horse-race" coverage that focuses on rival candidates' performance in polls rather than in-depth studies of their stands on issues or their records of service. Media outlets are also criticized for oversimplifying stories, emphasizing the human suffering caused by a natural disaster or a deadly conflict rather than exploring the conditions underlying the impact of the disaster or the bitterness of the conflict.
The business nature of the media also introduces some kinds of bias, not in terms of slanting coverage but in terms of story selection and timing. Major media outlets are subject to what has been called "profit bias," which is the need to generate advertising revenue, which means news editors choose stories that will attract viewers or readers. The need to draw an audience can persuade editors to emphasize attention-getting stories, such as sensational crime stories or lurid scandals. If the editors find that these stories do attract an audience, they will continue to run them, crowding out news that might be more important to government, the economy, or society. In a highly competitive industry that prizes being the first to break a story, journalists also exhibit a speed bias. The desire to scoop the competition by being the first to publicize a story can sometimes lead to errors either of fact or of adequate editorial care. For instance, in 2017 members of a CNN investigative reporting team ran a story that did not have a sufficient number of sources to meet the network's editorial standards. When that became known, CNN withdrew the story from its website, and the three journalists involved resigned.
Most media outlets express opinions on major issues and have commentary sections or shows. These are often presented in a way that is separated from factual reporting, though since the 1990s the distinction between reporting and opinion has become more blurred. Many newspapers have editorial sections that include opinions from their editors and op-ed pieces, or opinion pieces, on various issues. A section devoted to letters to the editor allows readers to communicate with editors (and to other readers). Radio and television news programming is sometimes less transparent about what is presented as news and what is given as commentary or opinion. In addition, these outlets do not always provide a public forum for viewer feedback like a newspaper's letters to the editor. Online news sources—including those of traditional media such as newspapers and TV news stations—typically allow visitors to comment on the stories they post, but the possibility of commenting anonymously can affect the nature of that feedback.
Over time, public perception of whether mass media is truthful has eroded. In a 1956 study two-thirds of Americans believed that newspapers delivered fair and accurate coverage of events. Six decades later, a similar poll found two-thirds of Americans thought news stories were inaccurate and unfair.