Modeling time-series count data: The unique challenges facing political communication studies
Introduction
Political communication scholars commonly study an outcome that is a count variable, such as the number of stories printed on a subject in a given time frame (Flemming et al., 1997, Fogarty, 2005, Peake and Eshbaugh-Soha, 2008, Rhee, 1996, Sellers, 2000, Ura, 2009). Since the unit of analysis is a day, week, or month of coverage, these data often are time dependent as well. Many studies of such data focus primarily on specifying a reasonable time series model as if these outcomes were normally distributed, ignoring the count aspect of these data. Certainly, if we were limited to addressing only one of these issues, time dependence is the more important, as it speaks to non-independence of observations and more importantly the dynamic form of causality in time dependent data. (In other words, an input can have a consequence on the outcome many time periods into the future.) However, substantial developments have been made in the creation of Poisson models for time dependent data (Brandt et al., 2000, Brandt and Williams, 2001, Blundell et al., 2002, Schwartz et al., 1996), implying that models of media outcomes can handle both time ordering and the proper distribution for the outcome.1
In this paper, we illustrate how to apply time-series count models to media outcomes by replicating several important studies in the field of political communication. In so doing, we accomplish two things: We show how several results change under this new specification, and we illustrate how to interpret quantities in a count model for time series data. We begin this paper by describing the background on research about media event counts. Second, we describe various approaches to modeling time series of event counts, including the Poisson autoregressive model and its properties. Next, we conduct three replication analyses, describe how model-sensitive results are in each, and illustrate the best way to interpret these results. The three replications revisit Flemming et al.’s (1997) analysis of news stories about church and state issues, Peake and Eshbaugh-Soha’s (2008) study of television coverage of energy policy, and Ura’s (2009) study of newspaper coverage of homosexuality. We conclude by discussing the implications of our results for the practical researcher.
Section snippets
Background on models of event counts in political communication
As with most social science research, the sophistication of political communication analysis has been increasing over the past few decades. This includes an interest in modeling media count data over time. Instead of relying on descriptive measures alone, scholars are increasingly trying to systematically explain variations in coverage of political phenomena over days, months, and years (Flemming et al., 1997, Fogarty, 2005, Peake and Eshbaugh-Soha, 2008, Ura, 2009). At the most basic level,
Approaches to modeling time-series count data
Within the literature on political communication, most of the empirical models take some approach that handles either the count aspect of the data or the time series aspect. To illustrate how inferences and forecasts can differ among plausible models, our paper refits several past models from this subfield using four models that are commonly used for analyzing count data in political communication research. The first such model is transfer function analysis (Box and Tiao, 1975, Box et al., 2008
Replication analyses
In this section, we reanalyze Flemming et al.’s (1997) study of church and state coverage, Peake and Eshbaugh-Soha’s (2008) study of energy policy coverage, and Ura’s (2009) study of national coverage of homosexuality. We choose these three studies because they all have made significant contributions to the study of political communication, helping to establish the substantial effect that presidential and Supreme Court actions have on the news media’s agenda. Further, these studies all use
Implications for the practical researcher
Political communication scholars over the past decade have enhanced our understanding of the press and politics by augmenting qualitative and cross-sectional studies with time-series studies of media coverage of political phenomena. By adding macro-level explanations of press attention to politics and government to micro-level explanations, the richness of political communication theories grows. In studies where time defines the unit of analysis, scholars have rightfully begun to incorporate
Acknowledgments
For helpful assistance, we thank Keith T. Poole, Richard L. Vining Jr., and several anonymous reviewers. For sharing code, we thank Patrick T. Brandt. For sharing data used in this paper, we thank B. Dan Wood, Joseph D. Ura, Jeffrey S. Peake, and Matthew Eshbaugh-Soha. For additional data shared, we thank Tobey Bolsen and Patrick Sellers. Complete replication data and code are posted on Monogan’s Dataverse: http://hdl.handle.net/1902.1/16677.
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