“PUBLIC OPINION”: bibliometric analysis for the systematization of trends (1945-2016)

This paper sought to identify trends in the research of “public opinion”, allowing for an analysis of the field in a systematized manner. We carried out a bibliometric study of 2,536 articles selected from the Web of Science . With the aid of the CiteSpace software , we aimed to characterize the research front and intellectual base of the field. In relation to the research front, we noted an increase in publications between 1945 and 2016; we identified Robert Shapiro and Lawrence Jacobs as the most productive authors; among the most referenced articles were those by Gamson and Modigliani (1989) and Page and Shapiro (1983). We also identified that the majority of articles are: by North American authors; were published in the journals Public Opinion Quarterly and Sotsiologicheskie Issledovaniya ; are indexed in the categories of Government & Law and Political Science and represented by the keywords public opinion , attitudes , support , policy , the United States and politics . With regard to the intellectual base in the journal co-citation network, the Public Opinion Quarterly , American Political Science Review and American Journal of Political Science stood out . In the author co-citation network the most prominent authors were Page and Zaller. In the analysis of the references co-citation, the most frequent texts in the network were by Zaller (1992) and Page and Shapiro (1992). We concluded that scientific output on public opinion is on the increase, with a dynamic expansion of the research front and intellectual base .

data mining, selection, organization and analysis of the material that will constitute the corpus of the research (Figure 1). According to the framework of bibliometric analysis presented, the first procedure for the operationalization of research was the choice of Web of Science by Thomson Reuters (Principal Collection of the Web of Science), since it is among the most complete and reliable sources for organizing scientific production in accordance with rigid standards of selection, being used by different scholars as a reference (Pinto, Serra, & Ferreira, 2014). In the search, we used the expression "public opinion" or "public opinions" in brackets in the "title" field, that is, the term had to be in the title of the articlewhereby the article would follow the central theme in its content. We only considered documents in the form of scientific article and in all the years available in the database -1945 to 2016. Moreover, no specific area was demarcated. At the end of the filtering, we found 2,536 articles.

Stages
Next, the other steps detailed in Figure 1 were followed. It is worth highlighting that by using specific software, bibliometric studies allow the researcher to deal with a large volume of information, which would be difficult implementing other analysis procedures (Pinto et al., 2014). To organize the references, we used EndNote (Free EndNote Trial). Nevertheless, in this article, we mainly used CiteSpace software for network presentations (Chen, 2006). In the analysis, we adopted the procedures described by Chen (2004Chen ( , 2006 regarding the use of CiteSpace to view intellectual turning points (Chen, 2004) and understand emerging trends and transient patterns (Chen, 2004) in a general manner. Important characteristics of CiteSpace use refer to the concepts of research front and intellectual base. Research front is the "emerging thematic trends and surges of new topics" (Chen, 2006, p. 362) and the intellectual base is represented in CiteSpace by the co-citation networks. CiteSpace also allows for the presentation of clusters as the figure illustrates. Chen (2006) demonstrates that through citations and co-citations we can discover the research trends on public opinion, the research front and the intellectual base of the field. In this way, the researcher may view the existing relations in his/her research field.

Results and discussion 4.1 Research front (sample of articles from Web of Science)
Considering the period from 1945 to 2016, we found an initial phase with few publications: two articles were published in 1945, one in 1946 and another two in 1947. A progressive and continuous growth in publications with focus on the term "public opinion" only began in 1956, as shown in Figure 2. In the Figure, we also include a graph in which the total of years was divided into 4 intervals of 17 years eachother analyses are shown later considering these timeframes. Despite some oscillations from one year to another, with increases followed by reductions and new increases, the overall trend was of growth, particularly from 2011 onwards. We noted that at the end of the 1990s there was a slight tendency towards fewer articles, going from 67 in 1998 to 45 in 1999, 42 in 2000 and 35 in 2001with growth returning in 2002 (43 articles) and 2003 (48 articles). We noticed that 2015 was the year with the most articles, totaling 175. In 2016 (research was carried out on September 19, 2016) 114 articles have already been published, exceeding the total number of articles published in 2014 and 2013. These numbers reveal that research on the theme of "public opinion" is on the increase, and is therefore of interest to various researchers. The increase in the number of articles opens space for the diversity of areas, interests and correlated research themes, stressing the plurality of sense attributed to "public opinion", as mentioned by Figueiredo and Cervellini (1995) when addressing the concepts of the term. These are the first indications that the research front of publications on public opinion is quite diversified in theoretical and methodological terms. Nevertheless, the fact the theme is of interest to various areas is not new, having been strengthened by articles that deal with elections (Campbell, 1960;Mueller, 1973), the effects of public opinion on public policies (Page & Shapiro, 1983;Burstein, 2003) and mass media (Lippmann, 1922;Zaller, 1992). The first two articles found in this search address the issue of the statistical and mathematical measurement of public opiniontext Mann (1945) is indexed in the category of Statistics & Probability and Mathematics, in Web of Science, and Hyman (1945) is indexed in Psychology.
In the researched samples, the pioneering works were On a problem of estimation occurring in public opinion polls, by Mann (1945), published in the journal Annals of Mathematical Statistics, and Community background in public opinion research, by Hyman (1945), published in the Journal of Abnormal and Social Psychology. Both are from 1945 (the first year available for searching Web of Science). Mann (1945) discusses properties of voter number estimation, considering a survey performed in the molds of simple random or stratified sampling. Also along the lines of quantitative studies on public opinion, Hyman (1945) identifies advances and shortfalls in the field of research on public opinion at that time.  1945 1946 1947 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 1945-1962 1963-1980 1981-1998 1999-2016 Next, we sought to identify, among the 2,536 sample articles (citing articles), which authors had published the most, especially the existence of authors who have published more than one article from those selected in the samplethese authors are important to demarcate the research front of the field. As such, Table 1 highlights the authors who have published seven or more articles, regardless of the authorship order, also presenting the article most cited in Web of Science (WoS)for each of the authors and the journal in which it was published. The Nº column denotes the total of articles published per author, and the number of citations is also indicated under Cit., including the most cited article. It is worth noting that the 11 authors shown in Table 1 have articles they co-authored amongst themselves. In this sense, the sum of their publications is 86 articles. Given a sample of 2,536 articles, the result suggests a fragmentation in the scenario of authors being published on this issue, that is, there are many researchers interested in the theme and reflecting on it, albeit without a central author in the field's research front. However, it is worth mentioning that the authors shown could have other articles and texts on the theme not considered because they are not indexed in Web of Science-Main Collection.
Even without authors who comprise a divergent volume of publications, we can highlight, from Table 1, the authors Robert Y. Shapiro (12 articles) and Lawrence R. Jacobs (10 articles), who have co-authored articles. Shapiro is a professor and researcher linked to the Department of Political Science at Columbia University. He is a specialist in American Politics, working on the themes of: interests of public opinion, policy formulation, political leadership, and means of communication and application of statistical methods. He is also co-author and co-editor of several books and articles published on these themes, such as the Oxford Handbook of American Public Opinion and the Media (edited with Lawrence R. Jacobs, Oxford University Press 2011). It should be stressed that Shapiro has been published on this subject for many years, as author and co-author, with articles spanning from 1983 (Page & Shapiro, 1983) to 2011 (Shapiro, 2011) and 2013 (Newport et al., 2013). Jacobs also has many years of experience in researching the theme, with special mention for his work in the 1980s (Jacobs & Shapiro, 1989) and after 2010 (Jacobs & Mettle, 2011).
In the above list of authors that have published the most, it is important to stress that they are all linked to North American institutions. Of the ten institutions of learning that most publish, all are North American, with special mention for University of California, University of Michigan, University of North Carolina, University of Wisconsin, Columbia University and Florida State University. In the light of this, we also sought to analyze the countries with the highest number of publications (this analysis only considers the country of the first author). As shown in Figure 3, the United States is far ahead in terms of the number of publications on "public opinion". Although other countries have published on the theme, such as England (116 articles), Canada (92 articles), Australia (59 articles), China (44 articles), Germany (42 articles), Russia (4 articles), Holland (36 articles), France (35 articles) and Spain (32 articles), the number of North American articles is far superior to the other countries (914 articles), which is illustrated by the size of the blue sphere in Figure 3. This result corroborates the assertion by Figueiredo and Cervellini (1995) on the prominence of North American research on the theme of public opinion, especially in its study and measurement. Therefore, results prove the predominance of the United States on the public opinion research front.
From the United States (the country with the highest number of articles), the most cited works were by the authors, Gamson, Page and Nowak, with their respective articles: Gamson and Modigliani (1989), with 1,108 citations; Page and Shapiro (1983), with 499 citations; and Nowak, Szamrej and Latane (1990), with 320 citations. As per the analysis that will be described later, these articles appear among the 10 most cited from the sample (citations in Web of Science). Also in relation to the United States, the first article found in the search was published in 1947: Public opinion measurement as an instrument in public health practice (Calver & Wingo, 1947), in the American Journal of Public Health and the Nations' Health.
Continuing the analyses of the articles from the sample, in Table 2, we highlight the 10 most cited articles (number of citations informed through Web of Science). Apart from the title of the articles, Table 2 shows the authors (Citation), the journal in which the article was published and the number of citations (Cit.). Table 2 The 10 most cited articles  from the sample of 2,536 articles (citations in Web of Science) Title of article Citation Cit. Media discourse and public-opinion on nuclear-power: a constructionist approach Gamson andModigliani -1989 1108 Effects of public-opinion on policy Page and Shapiro -1983 499 Spiral of silence -theory of public opinion Noelle-Neumann -1974 327 From private attitude to public-opinion: a dynamic theory of social impact Nowak, Szamrej and Latane -1990 320 Structure and consistency in public-opinion: the role of core beliefs and values Feldman -1988 281 Public opinion toward immigration reform: The role of economic motivations Citrin, Green, Muste and Wong -1997 262

Gender gaps in public opinion about lesbians and gay men
Herek -2002 229

Issue frames and group-centrism in American public opinion Nelson and Kinder -1996 211
What moves public-opinion Page, Shapiro and Dempsey -1987 211 The impact of public opinion on public policy: A review and an agenda Burstein -2003 207 Among the results, it is worth noting that 4 of the 10 most cited articles are from the 1980s and focus on themes such as the effects of public opinion (Page & Shapiro, 1983;Page et al., 1987), media and public opinion (Gamson & Modigliani, 1989) and the relation between beliefs, values and public opinion (Feldman, 1988). The oldest article is the Spiral of silence: theory of public opinion, by Noelle-Neumann (1974), published in the Journal of Communication. In it, the German political scientist puts forward an often cited theory on public opinion and mass communication, in which he argues that people are afraid of isolation (of being excluded) and therefore prefer not to express their opinions when they are contrary to the majority's (particularly on controversial themes).
The most cited article by far is Media discourse and public-opinion on nuclear-power: a constructionist approach, by Gamson and Modigliani (1989) published in the American Journal of Sociology. This article studies the relation between media and public opinion through an analysis of the discourse on nuclear energy over more than three decades. The intention is to understand the formation of public opinion on nuclear energy (Gamson & Modigliani, 1989). Regarding the journals in which the 10 articles were published, we noted that the majority came from areas related to political science; the themes are also linked to these areasit is the case of those that deal with influence of public opinion on political decisions (Burstein, 2003;Page & Shapiro, 1983), on the relation between media and public opinion, the impacts of public opinion, the relation of public opinion to beliefs and values, as well as looking at themes such as immigration and homosexuality, among others. Therefore, we reiterate that our analysis of the research front showed the existence of a plurality of themes, approaches and empirical research on public opinion, with a strong presence of themes related to the area of political science. In addition, on the research front, the article by Gamson and Modigliani (1989), on media and public opinion in the case of nuclear energy, stands out.
Next, we list the most frequent journals in the 2,536 published articles from the sample. This assessment was aimed at identifying any journal that was notable for the publication of articles on public opinion. In the list, we found a very high number of journalsmore than 960 demonstrating that the publications are disperse, being distributed between different journals, focused on wide-ranging areas (including political science, communication, economics, history, environmental sciences, health sciences, technology, engineering, literature, international relations, public policy, demography, public administration, business and others). Considering the journals from Table 3 Table 3 shows that the first 10 journals represent just 16.76% of publications, showing that the articles from the sample came from a very large number of journals. Nevertheless, the journals with the highest number of articles demonstrate a relative concentration of research on public opinion in areas linked to political sciences (American Journal of Political Science, Journal of Politics, American Political Science Review and Political Research Quarterly), or are journals specialized in public opinion, such as Public Opinion Quarterly and the International Journal of Public Opinion Research. As such, these are the characteristics of the research front in terms of journals.
In terms of the sheer number of articles, the Public Opinion Quarterly stands out. It has a high impact factor, was created in 1937 and is part of the Oxford University publications. The journal's publications deal with theoretical contributions to communication research, public opinion analysis and the investigation of methodological questions (Public Opinion Quarterly, 2016). In this journal, the most cited article was Gender gaps in public opinion about lesbians and gay men by Herek (2002), with 229 citations, and the oldest article was Public-opinion and national-security policy by Almond (1956). Herek (2002) analyzed the disparity in attitudes among heterosexuals in relation to homosexuals. The results showed that heterosexual men had less favorable attitudes towards homosexuals in terms of gender issues. The author's research fits the orientation of opinion surveys. Almond's research (1956) concerned public opinion and national security.
In Figure 4, we show the distribution of articles by categories in Web of Science (only areas of frequency equal to or greater than 44 appear in the figure). Indexation by categories shows that the theme of public opinion is multidisciplinary, even though it is mostly associated with Government and Law (844 articles), followed by studies in the areas of Political Science (780) and Communication (353). It is noteworthy that apart from the areas of knowledge cited, there are publications in various other areas of the sciences, such as medicine, art, cultural studies, international relations, education, environmental sciences, statistics, mathematics, business, demography, geography and ethics. The co-occurring subject above is presented in the time zone format (Chen, 2006), showing, apart from frequency of articles (marked by the size of the circles), the trend for field expansion (marking the positions in which the first article from each area was published). It demonstrates that the research front has expanded and today forms a multidisciplinary field (exact and earth sciences; human sciences; agrarian science engineering and applied social sciences), as Figueiredo and Cervellini (1995) also point out.
In general, we noticed that the texts already presented also stand out in this aspect, such as the article by Page and Shapiro (1983), which is indexed both in the Government & Law category, and also in Political Science. The article (Communication) by Gamson and Modigliani (1989), the most cited from our sample, also appears, as well as the classic article by Noelle-Neumann (1974). It is worth mentioning that the articles indexed in the category of History are little citedthe most cited article in this category was referenced only 32 times. Next, we will analyze the keywords used (co-occurring keywords), shown in Figure 5. The most frequent keyword was public opinion, used 333 times (which was expected given the field demarcation). After it came attitudes (235), support (126), policy (118), among others, as per Figure 5 and Table 4. The highlighted words comprise the subject areas of the research front and, therefore, indicate focuses and trends in public opinion research (in the network those with frequency of at least 20 times were highlighted). Table 4 shows the frequency of keywords and the most cited article that used the respective keyword. We noted that the words and articles again demonstrate the plurality of the field and the multiplicity of research carried out from different study perspectives. In the selection of the articles published, besides the natural predominance of the term 'public opinion' (333 times)the theme of this paperwe observed a network of words that could be connected to more frequent categories of the articles. Information, news and media, for example, are strongly related to the area of Communication. And policy, election, representation, democracy and politics are common terms in Political Science studies, as well as Government and Law. Attitude is one of the most used expressions among the keywords (235 times) and may be linked to studies from different areas and categories. Perhaps it is used so much due to its relation to the term opinion, taken to mean a personal attitude involving choices (Borges, 2014;Mateus, 2008). Other keywords indicate conceptual studies on public opinion, its impacts (measurement) or "public opinion survey", such as support, knowledge, model, dynamics, behavior, impact and perception. Other keywords suggest more specific empirical research contexts, such as war, immigration and foreign policy. It is worth noting that the term United States is the fifth most used keyword, linked to the fact that it is the country with the highest number of studies on public opinion, has a tradition of carrying out surveys in this area and is where the authors publish the most.
It is important to stress that there are different uses of the public opinion concept. According to Mateus (2008) and Figueiredo and Cervellini (1995), the confusion at the concept of the expression 'public opinion' is also tied to the association it has with opinion surveys. Figueiredo and Cervellini (1995), Habermas (1996) and Borges (2014) claimed that simplistic associations hinder the conceptual understanding of public opinion, since the emergence of opinion poll institutes was guided by an empiricist paradigm, leading to an abandonment of the fundamental concepts of the philosophical-political thinking that founded the concept of public opinion at the beginning of the 19th century. In the context of this debate, Perrin and McFarland (2011) argue that both public opinion researchers and their critics are correct in some aspects. Therefore, discussion of this impasse should entail a revision of the nature of public opinion in order to use research techniques suited to the purpose and to adopt processes for the critical assessment of results.

Intellectual Base for the Research into Public Opinion Field
Continuing with our analysis, we will now evaluate the co-citation networks of journals, authors and references (documents). To broaden our understanding of the journals, through CiteSpace, the journal co-citation network was created-in this case, journals in which articles from the sample were published are not shown, but rather the journals co-cited by the 2,536 articles analyzed (Figure 6). Among the most prolific journals, the oldest reference to the theme is from the American Journal of Sociology, with a citation dating back to 1945. The most recent reference is from Political Research Quarterly, 133, whose first citation was from 1996. In Figure 6, the dark blue colors indicate articles with the oldest references (before 1960) -American Journal of Sociology and Public Opinion Quarterly. Articles referenced predominantly at the end of the 1990s are in yellow (Political Research Quarterly).
Of the journals that feature the most, it is important to remember that Public Opinion Quarterly was already mentioned in this article. American Political Science Review is also a significant journal for research on political science, including publications in the areas of political theory, American politics, public policy, public administration, comparative politics and international relations (American Political Science Review, 2016). It is a publication from the American Political Science Association (APSA) and has been continually edited for 110 years (since 1906).
With regard to the journal co-citation network, the intellectual base of the field is marked by the journals Public Opinion Quarterly, American Political Science Review, American Journal of Political Science and Journal of Politics. Despite the presence of journals on different themes and areas of knowledge, the categories of Government and Law and Political Science are those that feature in the magazines with the highest number of articles on public opinion.
In relation to author co-citation networking, we identified the authors who comprise the intellectual base of the field. As shown in Figure 7, through the wide circumference of knots for the theme of public opinion, the highest number of references in the co-citation network comes from the authors, Benjamim I. Page (227) and John R. Zaller (205). Colors also denote the authors who were referenced in different periods of timeblue for networks of co-citations from the 1940s and 1950s, with special mention for G. Gallup and W. Lippmannclassic authors in the field who maintain a high number of citations for their work till this day. Phillip. E. Converse is shown in the green part of the network, with work dating back to 1960s, as is Robert S. Erikson, in the 1970s. The authors Page and Zaller are more recent, marked by their work in the 1980s and 1990s respectivelythe fact they are more cited could be interpreted based on Chen (2004Chen ( , 2006, who points out that recent texts tend to be cited more and therefore contribute to the increased number of citations. To bring the intellectual base to life, we prepared a brief profile of two of the main authors. Benjamin I. Page is a professor in the Department of Political Science at Northwestern University (USA). Apart from themes related to public opinion, their research is focused on American politics, internal and external policy and economic and political inequality. Among their best known works are: The rational public: Fifty years of trends in Americans' policy preferences, Effects of public opinion on policy and What moves public opinion?all three in co-authorship with Shapiro. John Zaller is a professor in the Department of Political Science at University of California, Los Angeles (USA). His work covers politics and public opinion. He is best known for his book published in 1992 -The Nature and Origins of Mass Opinion. Other works worth mentioning include: Journalists and Citizens Shape the News; The American Ethos, Public Attitudes Toward Capitalism and Democracy (in co-authorship with Herbert McCloskey).
To better understand the evolution of the field, we evaluated the co-citation networks dividing the period analyzed into 4 intervals of 17 years, as per Figure 8. (a) 1945-1962 (b) 1963-1980 (c) 1981-1998 (b) 1999-2016 Figure 8. Author co-citation network in 4 periods.
Here we visually represent the expansion of the field in terms of knots and links, demonstrating the growth of the intellectual base in research into public opinion. We noted that authors such as Lippmann and Lazarsfeld, who were significant in the field in the periods from 1945-1962 and 1963-1980, were no longer prominent in the following periods. On the other hand, Converse, for example, emerged as an important author in the period from 1963-1980 and remained prominent until the end of the time period covered by the sample. Zaller appears prominently only in the period from 1981-1998, being the most cited author in the co-citation period, at the start of the period spanning 1999-2016. Page, meanwhile, already features in the period from 1981-1998, and continues to do so in the following period. As such, in an overall analysis of all the years, Page is the author with the greatest frequency, with Zaller in second place.
Finally, we use another central network to help comprehend the field: document (reference) co-citation network -that appear in the cited references from our sample (2,536). In Figure 9, we can see that there are various sub-fields of publications on public opinion, carefully demarcated by the predominant literature in each decade: 1950s and 1960s (in blue), 1980s (in green), 1990s (in brown) and 2000s onwards (in yellow).
Through the network from Figure 9, along with Table 5, we can see that the most cited text in the field's intellectual base is the book The Nature and Origins of Mass Opinion, by Zaller (1992). In second place is The Rational Public: Fifty Years of Trends in Americans' Policy Preferences, by Page and Shapiro (1992). The texts by Page and Shapiro (1983), Apter (1964), Campbell (1960) and Converse (1964) are also among the most relevant. Table 5 gives emphasis to the most cited documents in the network, in the book or article format and the total number of citations (Cit.).  Zaller -1992 Cambridge University press 158 The rational public: fifty years of trends in Americans' policy preferences. Page and Shapiro -1992 Chicago: University of Chicago Press 93 Effects of public opinion on policy Page and Shapiro -1983 American Political Science Review 80 An economic theory of political action in a democracy Downs -1957 The Journal of Political Economy 73 Ideology and discontent Apter -1964 Collier-Macmillan 69 Surge and decline: A study of electoral change. Campbell -1960 Public Opinion Quarterly, 61 The nature of belief systems in mass publics Converse -1964 In Zaller (1992) seeks to construct "a theory of the dynamics of public opinion, formalized in a statistical model to study the effects of media on behavior and political attitudes" (Mundim, 2009, p. 2) and this work -The nature and origins of mass opinionbecame a reference in debates on political communication, particularly studies on "the media's effects during electionsalthough it may also be broadly applied in any study of the Effects of the Media".
Among the most relevant works, there is also the text by Lippmann (1922), which critically discusses the legitimacy of public opinion, taking it to mean "the images from our heads". It puts forward the various limitations of public opinion, which, in fact, amounts to the opinion of few and not of a true "public". This position contrasts, for example, with that of the German philosopher Jürgen Habermas, who also appears in the author co-citation network. Habermas (1996) argues that the legitimacy of political decisions needs to go through the formation of rational public opinion. He claims that this opinion, which is formed through open and equal debate, in the mix of the public arena, is capable of influencing the political system therefore, it is also an idea of popular sovereignty in democracy. The contextualization of these authors' positions shows the range of differing and even opposing positions in the field of studies on public opinion. Finally, the other texts, apart from revealing different concepts of public opinion, also highlight two notable themes from the field -"mass" and politics.

Conclusion
Public opinion is a controversial concept associated with discussions on manipulation, surveys of opinion, potential for mobilization, influence over public policies, and other topics. Authors with different perspectives and from various areas of knowledge have debated the matter. In the light of this, this paper aimed at systematizing the trends in the field of research on "public opinion". To this end, we carried out a bibliometric analysis of 2,536 articles selected from the Web of Science by Thomson Reuters. Using the CiteSpace software (Chen, 2006), we sought to describe the research front and intellectual base of this research field.
In relation to the research front, we noted the evolution and growth of publications over a period from 1945 to 2016. In our sample, we identified Robert Y. Shapiro (12 articles) and Lawrence R. Jacobs (10 articles) as the most prolific authors. The most referenced articles were those by Gamson and Modigliani (1989), with 1,108 citations, and by Page and Shapiro (1983), with 499 citations. Besides this, the majority of articles were by North American authors (914 articles). The highest number of publications on the theme came from the journals Public Opinion Quarterly (128) and Sotsiologicheskie Issledovaniya (73 articles). The categories with the largest indexation of articles are Government & Law (844 articles) and Political Science (780 articles). The keywords that appear the most are public opinion (333), attitudes (235), support (126), policy (118), united-states (103) and politics (96).
It is important to consider that it was in the United States that research on public opinion was concentrated at the beginning of the 20 th century (Figueiredo & Cervellini, 1995), and it has maintained this prolonged tradition of studies in the area, given that the majority of the most cited authors and journals are North American. We also confirmed the multidisciplinary nature of interest in the concept, with a variety of authors, journals, categories, and keywords that indicate studies in different areas of knowledge.
In relation to the intellectual base, in the journal co-citation network the journals Public Opinion Quarterly (674), American Political Science Review (611) and American Journal of Political Science (501) stood out. The author co-citation network showed Page and Zaller as the most prominent authors, while it is important to note that there were significant changes in the central authors from the field over the period from 1945 to 2016. Finally, in the analysis of the references co-citation, the most frequently occurring texts in the network were Zaller (1992) (158) and Page and Shapiro (1992). Based on the intellectual base, we observed that a large number of journals, authors and articles can be a factor that contributes to difficulties in formulating a single, all-encompassing concept for the term public opinion, since, as discussed, the concept is multidisciplinary (Figueiredo & Cervellini, 1995).
Results show that scientific output on public opinion is on the increase and that the theme is plural (multidisciplinary) in its approaches, research areas and study subjects. The concept was defined at the end of the 19th century and beginning of the 20 th century, when the expression started to be used with its contemporary connotation (Borges, 2014), before developing and adapting to historical social, economic and transnational contexts, while also being the subject of debate, challenges, redimensioning and reverence for diverse research. It is also noted, as per Figueiredo and Cevellini (1995), that the most appropriate term would be "public opinions", given that there is no sole understanding that may be singularized for this expression. Finally, based on this bibliometric analysis, new theoretical and empirical research may be performed bearing in mind the continuous and dynamic expansion of the research front and the intellectual base.