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Non-ignorable abstentions in roll call data analysis Rosas, Guillermo Shomer, Yael Uploaded 07-02-2008 Keywords ignorability IRT model roll call data legislative voting Abstract How should we deal with abstentions in roll-call data analysis? Abstentions are very common in decision-making bodies around the world, and very often obey to a strategic rationale. Methods to recover ideal points from roll-call datasets -- such as Nominate and MCMC IRT -- are based on assumptions about the ignorability of the abstention- generating mechanism. However, the strategic character of abstentions makes the assumption of ignorability difficult to meet in practice. We discuss different abstention-generating mechanisms to understand the conditions under which they may be deemed ignorable, and extend the MCMC IRT model so as to incorporate information from abstention patterns into inference about legislators' ideal points.