Spatial models are ubiquitous within political science. Whenever we confront spatial models with data, we need valid and reliable ways to measure policy positions in political space. I first review a range of general issues that must be resolved before thinking about how to measure policy positions, including cognitive metrics, a priori and a posteriori scale interpretation, dimensionality, common spaces, and comparability across settings. I then briefly review different types of data we can use to do this and measurement techniques associated with each type, focusing on headline issues with each type of data and pointing to comprehensive surveys of relevant literatures—including expert, elite, and mass surveys; text analysis; and legislative voting behavior.


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