Experts and manifestos: Different sources - same results for comparative research?

This article examines scales of Left/Right and Progressive/Conservative positioning of political parties to assess the relative internal and external validity of expert-driven data vis-à-vis document-based data. First, I discuss issues of conceptual travelling and conceptual stretching. Second, I assess the internal validity of the expert- and document-driven scales. It appears that there is indeed a difference between expert and document driven scaling. Thirdly, I examine external validity by comparing different scales in relation to various political variables. The analysis shows that the scales, when used as variables in comparative political research, produce different effects. The conclusion is that – in addition to internal validity problems – the external validity of scaled variables generated from different types of data needs to be assessed more carefully than is often the case in comparative politics.