Response scale overstretch: linear stretching of response scales does not ensure cross-project equivalence in harmonised data

Abstract

Differences in the response-scale formats constitute a major challenge for ex-post harmonisation of survey data. Linear stretching of original response options onto a common range of values remains a popular response to format differences. Unlike its more sophisticated alternative, simple stretching proves readily applicable without requiring assumptions regarding scale length or access to auxiliary information. The transformation only accounts for response scale length, ignoring all other aspects of measurement quality, which makes the equivalence of harmonised survey variables questionable. This paper focuses on the inherent limitations of linear stretching based on a case study focusing on the measurements of trust in the European Parliament by the Eurobarometer and the European Social Survey—8 timewise corresponding survey waves in 14 European countries (2004–2018). Our analysis demonstrates that the linear stretch approach to harmonising question items with different underlying response scale formats does not make the results of the two surveys equivalent. Despite harmonisation, response scale effects are retained in the distributions of output variables.