The aims of this study were (1) to understand the extent to which offering or not offering a Don’t know option has the potential of contaminating survey data, and (2) to investigate the interaction between offering a Don’t know option and the verbalisation of scale points. Results from an experimental study with 196 online panel members indicate that empirical data sets can be contaminated if Don’t know options are not offered to respondents who are unable to to assess an object under study. The maximum extent of data contamination could not be determined because only one product category was examined. But the contamination for the less known fast food restaurant under study amounted to almost 20% of the data. Furthermore results show that using the typical Likert scale verbalisation of the middle point (“neither agree not disagree”) is often misinterpreted as a Don’t know option by respondents, thus increasing the risk of data contamination that cannot be corrected retrospectively. Practical recommendations for market researchers are derived from these results.
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