Proxy documents as a source of measurement error in the Comparative Manifestos Project

This paper considers the issue of document type diversity in the Comparative Manifestos Project (CMP). For many years the CMP has been collecting and coding a variety of documents, such as speeches, pamphlets, newspaper articles and leaflets, as manifesto proxies. By using previously unexplored archival material to perform controlled comparisons between different types of documents, this paper argues that the coding of such documents introduced considerable measurement error to party position estimates. Statistical analyses indicate that this measurement error is systematic rather than random as it is often manifested as centrist bias in parties' left–right position estimates. Consequently, the paper argues that random error correction methods cannot always correct for error attributed to the coding of proxy documents. The paper concludes with some recommendations for third-party users of the CMP data and documents and a plea to the CMP research team.

► We address the problem of document diversity in the CMP.
► Coding proxy documents introduces systematic measurement error.
► Systematic error appears as centrist bias in the L–R estimates.
► Random error correction methods cannot always correct this bias.
► We conclude that the CMP needs to replace some of its documents.