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Bartels. 1993. Messages received: The political impact of media exposure. APSR 87: 267-285.
Using survey data, political scientists have found that the media's effects on public opinion are "minimal." But this finding is at odds with the opinions of political campaign managers and the findings of experimental research (Iyengar and Kinder 1987). Why are political scientists unable to empirically demonstrate media effects on public opinion?
The problem with assessing media effects is to distinguish the effect of media exposure from the effect of previously held opinions and information. Using a Bayesian updating model allows the weighting of old and new information in people's current opinions. Only new information that contradicts prior opinions can produce observable opinion change in direct proportion to the strength (uncertainty) of the prior opinions.
Uses 1980 NES data where a panel of respondents was interviewed at three time points: right before the presidential primary season, between the end of primary season and the national nominating conventions, and during the first month of the general election campaign.
This repeated measurement allows Bartels to estimate the magnitude of measurement error using a variant of the Wiley and Wiley model, and then incorporates this measurement error into errors-in-variable parameter estimates. He then compares a simple OLS model's estimates with the errors-in-variables estimates.
The OLS estimates of media impact were substantially smaller than the errors-in-variables estimates of media impact, suggesting that the failure to account for measurement error was responsible for some of the "minimal effects" results of earlier studies.
Also, allowing for the effects of measurement error shows that opinions are much more stable over the course of a campaign season than previously thought. This means that new information from the media must compete with a much greater mass of prior information than the earlier studies supposed.
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Bartels, Larry (author) • American Politics • Public Opinion • Media Effects
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