Independently validating ideology measures - A look at NOMINATE and adjusted ADA scores

Measures of political ideology are central to abroad range of political science scholarship. However, despite extensive evaluation of the relative characteristics of vote-based ideology measures, little scholarship examines their validity independent of legislator behavior. Ideological validity is overlooked because all existing measures are action based. To address this gap, this article applies an instrumental-variables technique called FILTER to assess the validity of two important measures of legislator ideology: NOMINATE and interest-group ratings. The technique is also applied to investigate whether the measured action-based ideology suffers from agenda bias. The results show that the measures produce valid estimates of legislator ideology. Moreover, FILTER offers an important alternative to existing action-based measures of ideology that might be inappropriate for use in some settings, such as when the dependent variable being studied is also action based.