Читать книгу Setting the Agenda - Maxwell McCombs - Страница 23

The evidence reviewed here, plus many other field studies conducted around the world, corroborate a cause-and-effect relationship between the media agenda and the public agenda. The initial necessary condition for demonstrating causality is a significant degree of correlation between the presumed cause and its effect, a condition met by hundreds of agenda-setting studies worldwide.

A second necessary condition for demonstrating causality is time-order. The cause must precede the effect in time. Even the initial Chapel Hill study was careful to juxtapose the results of the public opinion poll measuring public concern about the issues of the day with the content of the news media in the weeks preceding the interviewing as well as with the days concurrent with the interviewing.²⁸ Evidence of agenda-setting effects in the two subsequent US presidential elections was based on panel studies. There were two waves of interviewing and content analysis during June and October in Charlotte during the 1972 presidential election, plus a third wave of interviews immediately following the election.²⁹ During the 1976 presidential election there were nine waves of interviewing from February to December and content analyses of local newspapers and national television news across the entire year in three different communities.³⁰ Both of these panel designs allowed detailed tests of the time-order involved in the relationship between the media and public agendas.

Other evidence of agenda-setting effects reviewed here from a variety of non-election settings also involves longitudinal research designs that allowed tests of the time-order involved in the relationship between the media and public agendas. The examination of the civil rights issue in the United States spanned twenty-three years.³¹ There are eleven replications of this type of single-issue analysis based on a 41-month period during the 1980s,³² and an intensive week-by-week examination of five individual issues in Germany during 1986.³³ Eight local issues were analysed, both in the aggregate and individually, in Louisville during an eight-year period.³⁴

All of this evidence about agenda-setting effects is grounded in the ‘real world’ – public opinion surveys based on random samples of the public and content analyses of actual news media. This evidence illustrates agenda-setting effects in a wide variety of situations, and it is compelling for the very reason that it portrays public opinion in the real world. But these réalité portraits of public opinion are not the best evidence for the core proposition of agenda-setting theory that the media agenda has a causal influence on the public agenda because these measures of the media and public agendas are linked with numerous uncontrolled factors.

The best, most unequivocal evidence that the news media are the cause of these kinds of effects comes from controlled experiments, a setting where the theorized cause can be systematically manipulated, subjects randomly assigned to various versions of this manipulation, and systematic comparisons made among the outcomes. Evidence from experiments provides the third and final link in the chain of causal evidence that the media agenda influences the public agenda, demonstration of a direct functional relationship between the content of the media agenda and the response of the public to that agenda.

Changes in the salience of defence preparedness, pollution, arms control, civil rights, unemployment, and a number of other issues were produced in the laboratory among subjects who viewed versions of TV news programmes that had been edited to emphasize a particular public issue.³⁵ A variety of controls ascertained that changes in the salience of the manipulated issue were, in fact, due to exposure to the news agenda. For example, in one experiment, subjects who viewed TV news programmes emphasizing defence preparedness were compared to subjects in a control group whose news programmes did not include defence preparedness. The change in the salience of this issue was significantly higher for the test subjects than for the subjects in the control group. In contrast, there were no significant differences between the two groups from before to after viewing the newscasts for seven other issues.

Bringing the cause-and-effect evidence of the laboratory up to date, two experiments investigated the agenda-setting effects of online news on personal agendas. One experiment compared the salience of international issues among readers of the print and online versions of the New York Times. Although there were stronger effects for the print version of the newspaper, subjects exposed to both versions were significantly different from a control group with no exposure to the New York Times. Opening the door to further exploration of the agenda-setting process, these experimenters argued that ‘contemporary incarnations of internet news are subtly, but consequentially, altering the way that the news media set the public’s agenda’.³⁶ Another experiment exposed participants to either a CNN newscast or the CNN news site and then measured their recall of the top stories and their rankings of issue importance.³⁷ The influence of television news was stronger than the impact of the web on viewers. Arguably, the format of web pages does not always contain distinctive salience cues about the media agenda.

In recent years, researchers have examined the agenda-setting effect of the news media outside of the lab, using field experiments. The advantage, in this case, is that the strong internal validity of the experimental design is matched with an equally strong level of external validity, for the results are measured in the real-world beyond the lab. The most well-known of these experiments was carried out by Gary King and his associates at Harvard University.³⁸ After three years of negotiations, he and his colleagues were able to recruit several news organizations to participate in the experiment, including well-known publications like The Nation and Huffington Post. Then, for a year and a half, between two and five of these outlets, in different combinations each time, volunteered to write simultaneous stories on one of eleven broad subjects, such as immigration, climate, and education. Each cluster of stories ran on these outlets in one of two consecutive weeks. The influence of these stories was measured by comparing outcomes in the ‘treatment’ week in which the cluster of stories ran with the ‘control’ week in which the cluster did not run. Each time they carried out this procedure, the researchers conducted an automated content analysis of Twitter chatter. Examination of the discussion of the chosen subject on Twitter found that there was more than a 60 per cent increase in the week after the stories ran. In his commentary on the experiment in Science, Matthew Gentzkow concluded that this study ‘provides one of the most rigorous and convincing data points to date on the agenda-setting power of media’.³⁹

While experiments are sometimes criticized as artificial situations, they provide vital complementary evidence for the agenda-setting role of the news media. A complete set of evidence for agenda-setting effects requires both the internal validity of experiments where the media and public agendas are tightly controlled and measured and the external validity of content analysis and survey research whose designs assure us that the findings can be generalized beyond the immediate observations at hand to larger settings in the real world. From both streams of evidence, a major contribution of agenda-setting theory is that it makes an explicit connection between specific media content and its effects among the public.