This work continues development and evaluation of a practical approach to a Design Research Methodology developed previously [1] and extended to practical application in [2] and concentrates on new empirical techniques for research evaluation. The methodological challenge was to seek, without preconceptions, a research approach that enabled insight into the unobservable organisation of designers' knowledge of guideline content. A number of designers' perceptions of the similarity between items in a collection of aerospace guidelines were collected using a triad comparison. An analysis was developed that combines nonmetric multidimensional scaling analysis with hierarchical cluster analysis in order to represent the inherent structure of the perceived similarity in this data. This structure was established by examining the groupings and linear dimensions formed in the analysis outputs and relating these to guideline content. The results, which are not directly related to the standard engineering guideline organisation, may be a new indication of designers' mental models; suggesting that perceived similarity ratings and the analysis methods we have developed could be a powerful tool in eliciting designers' knowledge in general.@en