Diagram Size vs. Layout Flaws

Abstract

CONTEXT: Previously, we have defined the notion of diagram size and studied its impact on the understanding of UML diagrams. Subsequently, questions have been raised regarding the reliability and generality of our findings. Also, new questions arose regarding how the quality of diagrams could be defined, and how it interacts with diagram size. GOAL: We pursue three goals. First, we want to increase the validity of our research by analyzing a substantially larger data set than before. Second, we broaden the generalizability of our results by including two more diagram types. Our main contribution, though, is our third goal of extending our analysis aspects of diagram quality. METHOD: We improve our definition of diagram size and add a (provisional) definition of diagram quality as the number of topographic layout flaws. We apply these metrics on 60 diagrams of the five most commonly used types of UML diagram. We carefully analyze the structure of our diagram samples to ensure representativeness. We correlate diagram size and layout quality with modeler performance data obtained in previous experiments. The data set is the largest of its kind (n = 156).RESULTS: We replicate earlier findings, and extend them to two new diagram types. We provide an improved definition of diagram size, and provide a definition of topographic layout quality, which is one more step towards a comprehensive definition of diagram quality as such. Both metrics are shown to be objectively applicable. We quantify the impact of diagram size and quality on diagram understanding. CONCLUSIONS: The overall results of previous studies are confirmed, while our previous recommendations for creating better diagrams are revised and refined.

Definition of the Criteria for Layout of the UML Use Case Diagrams

“…According to [12], a diagram element is any line, shape, or textual label that appears in a diagram and:…”

“…Adornments that can be moved include multiplicities, association names. According to [12], a diagram flaw is an instance of: 1) bends of lines that are considered flaws; 2) intersections that are considered flaws if they are visible and not a syntactic element of the language; 3) touching elements that are considered a flaw, unless they have close syntactic or semantic association; 4) sets of merged lines or aligned lines that are close together and are considered a flaw, unless they have the same type and share exactly one of their endpoints; 5) two flaws that are fused into one flaw, if they are very close together and caused by the same intersecting elements. Finally, the topological quality of a diagram is defined as the number of laws it contains.…”

“…According to [12], a flaw rate of a diagram should not exceed 0.5 and number of flaws should not exceed 15-20, otherwise a diagram flaw rate makes it less readable and some important information might get lost. By analysing the layout algorithms, it may be seen that only few of the algorithms can reduce the flaw rate to be less than 0.5.…”

“…There are also several works devoted to an understanding of diagrams. Harald Störrle demonstrated how the diagram size affected the diagram flaw rate in [12], the more elements diagram contained, the greater number of flaws there were. In another work [25], the author also demonstrated that the quality of the placement of diagram elements affected its understanding.…”