HUMAN-COMPUTER INTERACTION
SECOND EDITION
HUMAN-COMPUTER INTERACTION
SECOND EDITION
To answer the second question concerning the drawback of the pen (or typewriter) metaphor for word processing, we refer to the discussion on metaphors in Section 4.2.6. The example there compares the functionality of the space key in typewriting versus word processing. For a typewriter, the space key is passive; it merely moves the insertion point one space to the right. In a word processor, the space key is active, as it inserts a character (the space character) into the document. The functionality of the typewriter space key is produced by the movement keys for the word processor (typically an arrow
One principled approach to interactive system design which will be important in later chapters relies on a clear understanding early on in the design of the tasks that the user wishes to perform. One problem with this assumption is that the tasks a user will perform are often only known by the user after he is familiar with the system on which he performs them. The chicken-and-egg puzzle applies to tasks and the artefacts on which he performs those tasks. For example, before the advent of word processors, an author would not have considered the use of a contracting and expanding outlining facility to experiment easily and quickly with the structure of a paper while it was being typed. A typewriter simply did not provide the ability to perform such a task, so how would a designer know to support such a task in designing the first word processor?
The justification for such a guideline is that the more options (or controls, as the term is used in the quoted guideline) on a menu, the longer it will take a user to locate and point to a desired item. As we discussed in Chapter 1, humans chunk related information in the learning process and this can be used to increase the efficiency of searching. Grouping of related items in a menu can supplement this chunking procedure. But be warned! Remember the scenario described in the Introduction to this book, in which we fell victim to closely grouped menu items which had drastically different effects in our word processor. Saving and deleting files might be considered logically similar since they both deal with operations on the file level. But simple slips done in pointing (which are all too easy with track ball devices) can change an intended save operation into an unintended and dangerous delete.
Carroll refers to this real-life phenomenon as the task-- artefact cycle. He provides a good example of the task-- artefact cycle through the evolution of the electronic spreadsheet. When the first electronic spreadsheet, VisiCalc, was marketed in the late 1970s, it was presented simply as an automated means of supporting tabular calculation, a task commonly used in the accounting world. Within little over a decade of its introduction, the application of spreadsheets has far outstripped its original intent within accounting. Spreadsheets are now used for all kinds of financial analysis, 'what-if' simulations, report formatting and even as a general programming language paradigm! As the set of tasks expanded, new spreadsheet products have flooded the marketplace trying to satisfy the growing customer base. Another good example of the task-- artefact cycle in action is with word processing, which was originally introduced to provide more automated support for tasks previously achieved with a typewriter and now provides users with the ability to carry out various authoring tasks that they never dreamed possible with a conventional typewriter. And today, the tasks for the spreadsheet and the word processor are intermingled in the same artefact.
Selection From the above snippet we see the use of the word select where the choice of methods arises. GOMS does not leave this as a random choice, but attempts to predict which methods will be used. This typically depends both on the particular user and on the state of the system and details about the goals. For instance, a user, Sam, never uses the L7-METHOD, except for one game, 'blocks', where the mouse needs to be used in the game until the very moment the key is pressed. GOMS captures this in a selection rule for Sam:
In fact, the CCT rules can represent more complex plans than the simple sequential hierarchies of GOMS. The continuous activity of all production rules makes it possible to represent concurrent plans. For example, one could have one set of production rules representing the goal of writing a book, and another set representing the goal of drinking tea. These rules could both be active simultaneously, thus allowing an author to drink tea whilst typing. Despite this apparent flexibility, CCT is not normally used in this way. It is not clear why this is, except that CCT, like GOMS, is aimed at low-level, proceduralized goals -- that is, the unit task. It is reasonable that successive unit tasks be chosen from different activities: the author may delete a word, have a drink, do a word search, but each time a complete unit task would be performed -- the author does not take a drink of tea in the middle of deleting a word.
movement[Direction]
:= command[Direction] + distance + RETURN
command[Direction]:= known-item[Type=word,Direction]
* command[Direction=forward] := 'FORWARD'
* command[Direction=backward] := 'BACKWARD'
* command[Direction=left] := 'LEFT'
* command[Direction=right] := 'RIGHT'
processed in 0.007 seconds
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