Consistency is important in such information products that should be instantly ready to be used and that should provide simple use and learning in any context. It is often mentioned as one of the most important and desirable design goals because it is assumed to improve the user’s possibility for transfer of skills from one system to another (for example Shneiderman 1998, Nielsen 1993, Nielsen 2001). Still, user interfaces of today are often inconsistent (Nielsen 2001).
The concept of consistency is difficult - even a team of 15 experts was not able to produce a definition for consistency (Nielsen 1989). Grudin (1989) criticizes that design emphasis on user interface consistency directs the designers’ attention away from users and the context of use.
Consistency should be maintained, not only inside one product, but also between current and earlier systems (Nielsen 1993, 90).
User interface consistency is used in three interrelated senses: the internal consistency of a design with itself; the external consistency of a design with other interface designs familiar to a user; and an external analogic or metaphoric correspondence of a design to features in the world beyond the computer domain (Grudin 1989).
Smart phones have high number of functions and applications embedded in a compact form.
Hence Smart phones are sometimes also called feature phones. The high number of embedded functions leads to the challenge of designing an internally consistent user interface. Internal consistency of an interface design is a goal that can be achieved by the product designers. In its simplest, internal consistency means similar menus, terms and action patterns. A project team can improve internal consistency, for example, with style guides and design coordination tasks.
Internal consistency has positive effect on learning, ease of use and perceived quality.
External consistency of interface functions with functions of other interfaces familiar to the users is a far more difficult goal to achieve. External consistency can be achieved together with internal consistency if the interfaces are similar enough, but external consistency is often achieved at the expense of internal consistency. External consistency helps the user especially with the initial learning of the system by enabling the user to apply skills learned with earlier systems. When a product design adopts functions from other interfaces, the design team needs to decide which is more important, internal or external consistency (for example, Article I). This design issue is becoming an important issue in those design areas where new functionality is created by combining existing technologies. In the design of smart products the difficulty in achieving external consistency is also in the fact that the physical interfaces in the products are very different, for example a digital camera vs. a mobile phone, or a FM radio vs. mobile phone.
Nielsen (2001) lists the different levels of consistency expanding especially on the characterisrics of external consistency:
- the individual application - across a product family
- for all products released by a vendor
- for all products running on a specific computer - for the national computer industry
- internationally (for everybody, everywhere).
Correspondence of interface functions to familiar functions of the world beyond computing need to be analysed and designed when such metaphors, analogies, attributes or relations are applied that originate from non-computer interfaces. For example, when metaphor "scissors" is used in a drawing application.
The user is not able to understand different kinds of consistencies as well as a designer can.
According to Grudin (1989), where the designers see two applications with internally consistent but externally inconsistent interfaces, a user may see one internally inconsistent system.
6.2.1 Converging technologies
Question: What do you get when you cross a computer with a camera?
Answer: Computer (Cooper 1999).
When new technologies converge in smart devices or new functions are added, there is always the question how internal or external product consistency is affected by the changes, or which consistency should be maintained. For example, if a digital camera is embedded in a smart phone, how much of the design should be consistent with digital cameras and how much with mobile phones?
Sinkkonen (2001) notes that systematic development of learnability is essential for developing mobile devices that are easy to take into use. One problem solving strategy in learning to use a new system is analogical reasoning. Every user applied this strategy in the beginning of learning process. Analogical reasoning in product use means that the user uses his knowledge and mental models from another product as a source and by imitating that, solve the target problem (Thagard 1996). Analogical reasoning leads to results if there is enough consistency between earlier experiences and current learning task.
Most of the functions that are designed to a smart phone exist already in some other system. For example, email, browser, text editor and time management applications are familiar from desktop computers, and imaging capabilities from digital cameras. Due to the different user interaction elements and mobile phone capabilities the functions and applications very often need to be redesigned and simplified for a mobile phone user interface.
Although people are highly adaptive to new platforms, their previous experiences are, nevertheless, critical in shaping their expectations (Norman 1988). For example, a web browser on a new platform should behave in a reasonably similar manner as browsers in other environments, to achieve external consistency. On the other hand, few devices are built for a single application only; the browser should also work in harmony with the other applications to
guarantee internal consistency. The designer is often faced with a difficult design issue: how to weigh the different factors, or design constraints, that point in different directions.
It is precisely for this reason why consistency is a somewhat controversial design principle.
While it is generally agreed that consistency is a worthwhile goal to strive for (Preece et al.
1994, Shneiderman 1998), many authors (Grudin 1989, Grudin 1992, and Norman 1988) warn about the pitfalls of taking only one aspect of consistency as the guiding rule in design. The overall goal, of course, should be to produce a usable product.
Monk (2000) uses terms action-effect consistency, i.e. consistency in the effects of actions, and task-action consistency, i.e. consistency in the way actions relate to task goals. Action-effect consistency is the principle that if the user takes some action it should have the same effect whatever the context, i.e. the interface should be mode free. For example, double clicking on a word should have the same effect in all parts of the user interface. In small devices, such as mobile phones, there are inevitably several modes, because there can be a large number of commands but only few buttons to access those. The action of pressing a button will have different meanings depending on the mode the phone is in. This leads to action-effect inconsistency (modedness), which may be acceptable if the user is aware what mode he is in.
Task-action consistency is intended to result in transfer of training when learning the set of actions needed to achieve similar goals. Similar goals should require similar sets of actions to achieve them.
Function richness and redesign of applications are challenges for the design of new smart phones, for example, because:
- overall product complexity increases (new functions, new technology) - product size decreases (smaller and/or less interaction elements) - new technologies are embedded
- new technologies are merged together (new functions are created based on the opportunity that the merging of technologies provide)
- functionality is downscaled from desktop computers and other systems to mobile phones (familiar tasks are redefined and renamed)
In order to provide an externally and internally consistent user interface, the designer needs to have sufficient knowledge about:
- what kind of consistency is preferred?
- general and product specific design guidelines - design of other parts of the user interface
- dominant designs of similar functions and possible sources of transfer-learning
- the characteristic and opportunities of the new available technology, or about the merged new technologies.
The lessons from the development of three generations of communicators have shown that:
- basic and logical function consistency is required between systems. For example, certain functions of text messaging application needs to be available device independently in order to meet the user expectations and requirements.
- when the product is an integrated entity, the internal consistency is more important than external consistency. For example, users may not expect similar menu structures in a hand- held device and a desktop computer for the same application domain.
- even if the internal consistency is an explicit design goal, it is difficult to fully implement due to product's structure
- product should be inter-consistent enough to provide easy switch from one product to another.
6.2.2 Development organisation
Nokia launches several new phone models each year. Increasing number of the phone users are replacement users, i.e. users already have a mobile phone and they want to replace it with new one. Currently more than 70% of the customers are replacement users (Gartner 2001). The importance of inter-consistency is high, because users do not like learning new ways of using the new phone. Also the need for product consistency is high because users expect to be able to use new functions in the same as they use the familiar ones, and to quickly learn the new functions.
If the product development organisation succeeds in the product design, it creates a product that has external consistency with other products and internal consistency between the new functions and old functions. If usability engineering is insufficient there is a high risk that both internal and external product consistencies are poor.
A special challenge for the development organisation is to develop internally and externally consistent products in the multi-site development environment. External consistency can be difficult to maintain, even between company products, if the projects or whole teams are progressing in different continents. To develop a product with internal consistency is challenge for multi-site teams, where the individual designers of one team are located in different places.
The effect of insufficient usability engineering can be seen also on practical level of project performance. The following table (Table 6.1) lists some observations made in the development of smart phones and communicators.
Sufficient usability engineering Insufficient usability engineering Design creation Design and improve by iteration Late rework needed
Schedule No negative effect Timetable pressures due to rework.
Designer support
Faster problem solving, more confident about decisions in the form of decreased design uncertainty.
Awareness of the design goal.
Difficult to verify design decisions. No awareness when the design is "good enough". Design uncertainty.
Management support
Awareness of expected user problems. No realistic understanding about expected user problems.
Usability engineering costs
Can be predicted Not fully predictable due to costly changes and rework in the late development phases.
Table 6.1. Some comparisons on the effects of sufficient and insufficient usability engineering.