Introduction: Triangulation and the square-root law

This special issue of Electoral Studies evaluates the validity of expert, manifesto, and survey data on the positioning of national political parties. My purpose in this introduction is to examine the logic and limits of triangulation. In doing so, I make explicit the most important lesson of this special issue: improving the validity demands that one take full advantage of the information that is available. Even if observation is inherently biased, one can improve accuracy by comparing the observations that are biased in different ways.