The aims of this study were to (1) understand the extent to which offering or not
offering a Don’t know option has the potential of contaminating survey data, and
(2) 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 confirm that
substantial proportions of empirical data sets can be contaminated if Don’t know
options are not offered to respondents who do not feel competent to assess an
object under study due to lack of familiarity. 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
Practical recommendations for market researchers are derived from these results.