Design rulesDr. Yan LiuDepartment of Biomedical, Industrial and Human Factors EngineeringWright State University2Introduction Design Rules Rules that a designer can follow in order to increase the usability of the eventual products Can be supported by psychological, cognitive, ergonomic, sociological, economic or computational theories which may or may not have roots in empirical evidence Authority Whether the rule must be followed in design or it is only suggested Generality Whether the rule can be applied to many design situations or it is focused on a more limited application situation3Introduction (Cont’d) Types of Design Rules Principles Derived from knowledge of the psychological, computational and sociological aspects of the problem domains  Largely independent of the technology and dependent to a much greater extent on a deeper understanding of the human element in the interaction Abstract design rules, with low authority and high generality Guidelines Less abstract than principles and often more technology oriented Still general; important for a designer to know what theoretical evidence there is to support them Relatively higher authority than principles Standards Specific design rules; less important for a designer to know the underlying theory High authority and limited application4Introduction (Cont’d) Issues Conflicts between design rules The theory underlying the separate design rules can help the designer understand the trade-off for the design  The more general a design rule is, the greater the likelihood that it will conflict with other rules and the greater the need for the designer to understand the theory behind it When to use design rules within the design process Design rules would be most effective if they could be adopted in the earliest stages of the life cycle Requirement specification Some specific design rules are applicable only at later stages of the design life cycle e.g. Design rules on color vs. monochrome screens or two- versus three-button mouse depends on the particular hardware platform5Principles to Support Usability Categories Learnability  The ease with which new users can begin effective interaction and achieve maximal performance Flexibility The multiplicity of ways in which the user and system exchange information Robustness The level of support provided to the user in determining successful achievement and assessment of goals6Learnability (Cont’d) Predictability The degree to which user’s knowledge of the interaction history is sufficient to determine the result of his/her future interaction with it Degrees to which predictability can be satisfied The knowledge is restricted to the presently perceivable information, so that the user does not need to remember anything other than what is currently observable The knowledge requirement is so high that the user is forced to remember what every previous keystroke was and what every previous screen display contained (and even the order of each) in order to determine the consequences of the next input action Operation visibility How the user is shown the availability of operations that can be performed next  If an operation can be performed, then there should be some perceivable indication of this to the user The user should understand from the interface if an operation he/she might like to invoke cannot be performed Supports the superiority in humans of recognition over recall7Learnability (Cont’d) Synthesizability The ability of the user to assess the effect of past operations on the current state “Honesty” of the system The ability of the user interface to provide an observable and informative account of any change in the internal state of the system In the best circumstance, this notification should come immediately, requiring no further interaction by the user (immediate honesty) e.g. In a Windows system, to move a file from one directory to another directory, the user drags the visual icon of the file from the original directory to the destination directory where it remains visible At the very least, the notification should appear eventually, after explicit user directives to make the change observable (eventual honesty) e.g. In a command language system, the user would have to remember the destination directory and ask to see the contents of the directory in order to verify that the file has been moved successfully8Learnability (Cont’d) Familiarity  The correlation between the user’s prior knowledge and the knowledge required for using the new system e.g. When the word processor was originally introduced, the analogy between the word processor and a typewriter was intended to make the new technology more immediately accessible to those who had little experience with the former but a lot of experience with the latter Affordance  The appearance of an object suggests how it can be manipulated e.g. The shape of a door handle suggests how it should be manipulated to open a door; a key on a keyboard suggests that it can be pushed Effective use of the affordances that exist for interface objects can enhance the familiarity of the interactive system9Learnability (Cont’d) Generalizability  The ability of the user to extend his/her specific interaction knowledge to situations that are similar but previously unencountered Can occur within a single application or across a variety of applications e.g. In a graphical drawing package that draws a circle as a constrained form of ellipse, the user can generalize that a square can be drawn as a constrained rectangle One of main advantages of standards is that they increase generalizability across different applications within the same environment10Learnability (Cont’d) Consistency  The likeness in behavior arising from similar situations or similar task objectives Must be applied relative to something e.g. consistency in command naming, consistency in icon layout Many other principles can be considered as instances of consistency Familiarity can be considered as consistency with respect to past experiences Generalizability can be considered as consistency with respect to experience with similar applications on the same platform11Flexibility Dialog Initiative System pre-emptive The system initiates all the