Design researchers have long sought to understand the mechanisms that support creative idea development. However, one of the key challenges faced by the design community is how to effectively measure the nebulous construct of creativity. The social science and engineering communities have adopted two vastly different approaches to solving this problem, both of which have been deployed throughout engineering design research. The goal of this paper was to compare and contrast these two approaches using design ratings of nearly 1000 engineering design ideas paired with a qualitative study with expert raters. The results of this study identify that while these two methods provide similar ratings of idea quality, there was a statistically significant negative relationship between these methods for ratings of idea novelty. Qualitative analysis of recordings from expert raters' think aloud concept mapping points to potential sources of disagreement. In addition, the results show that while quasi-expert and expert raters provided similar ratings of design novelty, there was not significant agreement between these groups for ratings of design quality. The results of this study provide guidance for the deployment of idea ratings in engineering design research and evidence for the development and potential modification of engineering design creativity metrics.