Does metaphor increase visual language usability?

Previous research suggests that the graphical metaphor should increase the usability of visual programming languages (VPLs) by providing an instructional aid when learning to use the language. This paper describes three experiments which manipulated the degree of metaphor in VPLs. In the first, an implicit pictorial metaphor was added to a data flow language, while in the second and third an explicit explanatory metaphor was provided for diagrams showing computational concepts. In both cases, the performance of novices was compared to that of expert programmers, in order to judge the improvement in usability that might result from this instructional device. The resulting benefits of metaphor were smaller than expected especially relative to the advantage of experience. This suggests that metaphor may not be an essential component in the usability of VPLs.