HUMAN-COMPUTER INTERACTION
SECOND EDITION
HUMAN-COMPUTER INTERACTION
SECOND EDITION
Whereas it is true that direct manipulation interfaces make some tasks easier to perform correctly, it is equally true that some tasks are more difficult, if not impossible. Contrary to popular wisdom, it is not generally true that actions speak louder than words. The image we projected for direct manipulation was of the interface as a replacement of the underlying system as the world of interest to the user. Actions performed at the interface replace any need to understand their meaning at any deeper, system level. Another image is of the interface as the interlocutor or mediator between the user and the system. The user gives the interface instructions and it is then the responsibility of the interface to see that those instructions are carried out. The user--system communication is by means of indirect language instead of direct actions.
We can attach two meaningful interpretations to this language paradigm. The first requires that the user understands how the underlying system functions and the interface as interlocutor need not perform much translation. In fact, this interpretation of the language paradigm is similar to the kind of interaction which existed before direct manipulation interfaces were around. In a way, we have come full circle!
The second interpretation does not require the user to understand the underlying system's structure. The interface serves a more active role, as it must interpret between the intended operation as requested by the user and the possible system operations that must be invoked to satisfy that intent. Because it is more active, some people refer to the interface as an agent in these circumstances. We can see this kind of language paradigm at work in an information retrieval system. You may know what kind of information is in some internal system database, such as the British highway code, but you would not know how that information is organized.
Whatever interpretation we attach to the language paradigm, it is clear that it has advantages and disadvantages when compared with the action paradigm implied by direct manipulation interfaces. In the action paradigm, it is often much easier to perform simple tasks without risk of certain classes of error. For example, recognizing and pointing to an object reduces the difficulty of identification and the possibility of misidentification. On the other hand, more complicated tasks are often rather tedious to perform in the action paradigm, as they require repeated execution of the same procedure with only minor modification. In the language paradigm, there is the possibility of describing a generic procedure once (for example, a looping construct which will perform a routine manipulation on all files in a directory) and then leaving it to be executed without further user intervention.
Probably the most significant recent development in interactive computing is the World Wide Web, sometimes simplified to just the Web, or WWW. The Web is built on top of the Internet, and offers an easy to use, predominantly graphical interface
Whilst the Internet has been around since 1969, it did not become a major para-digm for interaction until the advent and ease of availability of well-designed graphical interfaces (browsers) for the Web. These browsers allow users to access multimedia information easily, using only a mouse to point and click. This shift towards the integration of computation and communication is transparent to users; all they realize is that they can get the current version of published information practically instantly. In addition, the language used to create these multimedia documents is relatively simple, opening the opportunity of publishing information to any literate, and connected, person. However, there are important limitations of the Web as a hypertext medium and in Chapter 16 we discuss some of the special design issues for the Web. Interestingly, the Web did not provide any technological breakthroughs; all the required functionality previously existed, such as transmission protocols, distributed file systems, hypertext and so on. The impact has been due to the ease of use of both the browsers and HTML, and the fact that critical mass (see Chapter 14) was established, at first in academic circles, and then rapidly expanded into the leisure and business domains. The burgeoning interest led to service providers, those providing connections to the Internet, to make it cheap to connect, and a whole new subculture was born.
Eager is also an example of an agent which has a clear embodiment. That is, there is a representation of Eager (the cat icon) in the interface. In contrast, consider Microsoft Excel which incorporates some intelligence in its sum (S) function. If the current cell is directly below a column of numbers, or if there is a series of numbers to the left of the current cell, the sum range defaults to be the appropriate cells. It is also clever about columns of numbers with subtotals so that they are not included twice in the overall total. As around 80% of all spreadsheet formulae are simple
Where does computing happen, and more importantly, where do we as users go to interact with a computer? The past 50 years of interactive computing show that we still think of computers as being confined to a box on a desk or in an office or lab. The actual form of the physical interface has been transformed from a noisy teletype terminal to a large, graphical display with a WIMP or natural language interface, but in all cases the user knows where the computer is and must walk over to it to begin interacting with it. After all the progress of interactive technology we are still pretty much stuck to the desktop.
This notion of predictability deals with the user's ability to determine the effect of operations on the system. Another form of predictability has to do with the user's ability to know which operations can be performed. Operation visibility refers to how the availability of operations which can next be performed is shown to the user. If an operation can be performed, then there may be some perceivable indication of this to the user. This principle supports the performance in humans of recognition over recall. If not, then the user will have to remember when he can perform the operation and when he cannot. Likewise, the user should understand from the interface if an operation he might like to invoke cannot be performed.
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