Mental models are poorly specified in three ways: in their defining criteria, their source and the bias to which they have been subjected. Literature from psychology, HCI and human factors sources was reviewed to determine the utility of 'mental models' as a design tool. The definitions and theories offered by key contributors to the notion of mental models were compared. Schematics, representing both the knowledge structures proposed in cognitive processing, as well as the layers of bias evident when forming or accessing mental models, were constructed. Fundamental similarities and differences in the use of this notion, as well as ambiguities in definition, were highlighted graphically. The need for specificity in the use of mental models was emphasised as essential for pragmatic application in design. The use of graphical comparison was proposed as a means of identifying the risk of bias and a means to categorise approaches to mental model research. Practitioner Summary: Mental models are considered significant in user centred design. To apply this notion pragmatically, its definition and methods of construction and access need to be sufficiently specified. This article offers a graphical method to compare existing research in mental models, highlighting similarities, differences and ambiguities.