Software Engineering-Interface Design Metrics

Although there is significant literature on the design of human/computer interfaces , relatively little information has been published on metrics that would provide insight into the quality and usability of the interface.

Sears suggests that layout appropriateness (LA) is a worthwhile design metric for human/computer interfaces. A typical GUI uses layout entities—graphic icons, text, menus, windows, and the like—to assist the user in completing tasks. To accomplish a given task using a GUI, the user must move from one layout entity to the next. The absolute and relative position of each layout entity, the frequency with which it is used, and the “cost” of the transition from one layout entity to the next all contribute to the appropriateness of the interface.

For a specific layout (i.e., a specific GUI design), cost can be assigned to each sequence of actions according to the following relationship:

       cost = [frequency of transition(k) x cost of transition(k)]

where k is a specific transition from one layout entity to the next as a specific task is accomplished. The summation occurs across all transitions for a particular task or set of tasks required to accomplish some application function. Cost may be characterized in terms of time, processing delay, or any other reasonable value, such as the distance that a mouse must travel between layout entities. Layout appropriateness is defined as

      LA = 100 x [(cost of LA - optimal layout)/(cost of proposed layout)]

where LA = 100 for an optimal layout.

To compute the optimal layout for a GUI, interface real estate (the area of the screen) is divided into a grid. Each square of the grid represents a possible position for a layout entity. For a grid with N possible positions and K different layout entities to place, the number of possible layouts is represented in the following manner :

        number of possible layouts = [N!/(K! x (N - K)!] x K!
As the number of layout positions increases, the number of possible layouts grows very large. To find the optimal (lowest cost) layout, Sears proposes a tree searching algorithm.

LA is used to assess different proposed GUI layouts and the sensitivity of a particular layout to changes in task descriptions (i.e., changes in the sequence and/or frequency of transitions). The interface designer can use the change in layout appropriateness, ΔLA, as a guide in choosing the best GUI layout for a particular application.

It is important to note that the selection of a GUI design can be guided with metrics such as LA, but the final arbiter should be user input based on GUI prototypes. Nielsen and Levy report that “one has a reasonably large chance of success if one chooses between interface [designs] based solely on users’ opinions. Users’ average task performance and their subjective satisfaction with a GUI are highly correlated.”
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