Building policy scales from manifesto data: A referential content validity approach

•MARPOR/CMP scales have been built by many methods, limiting their comparability.•A new method of component selection is developed from first principles.•The new method builds on the content validity of manifesto components.•Three scales are built, using different conceptions of political dimensionality.

Researchers using scales based on MRG/CMP/MARPOR's manifesto dataset face a bewildering array of different scales. The validation of these scales has tended to focus on external, convergent validity. The actual content of these scales has received less attention and the choice of the manifesto components which make up these scales has often been conducted by either opaque or questionable methods. This article develops a critique of existing methods of component selection and proposes a new method of component selection based on the covariance of components with ‘naïve’ provisional scales, which are refined in an iterative process. It uses this method to construct a set of comparable one (general left–right) and two (economic and social) dimensional scales – filling a gap in the existing body of scales that will allow researchers to compare dimensionality across models without inadvertently comparing different assumptions that underlie the construction of the scales.