HUMAN-COMPUTER INTERACTION
SECOND EDITION
HUMAN-COMPUTER INTERACTION
SECOND EDITION
One of these factors is our perception of depth. If we return to the hilltop scene there are a number of cues which we can use to determine the relative positions and distances of the objects which we see. If objects overlap, the object which is partially covered is perceived to be in the background, and therefore further away. Similarly,
The speed at which text can be read is a measure of its legibility. Experiments have shown that standard font sizes of 9 to 12 points are equally legible, given proportional spacing between lines [239]. Similarly line lengths of between 2.3 and 5.2 inches (58 and 132 mm) are equally legible. However, there is evidence that reading from a computer screen is slower than from a book [166]. This is thought to be due to a number of factors including a longer line length, fewer words to a page, orientation and the familiarity of the medium of the page. These factors can of course be reduced by careful design of textual interfaces.
So if learning information is aided by structure, familiarity and concreteness, what causes us to lose this information, to forget? There are two main theories of forgetting: decay and interference. The first theory suggests that the information held in long-term memory may eventually be forgotten. Ebbinghaus concluded from his experiments with nonsense syllables that information in memory decayed logarithmically, that is that it was lost rapidly to begin with, and then more slowly. Jost's law, which follows from this, states that if two memory traces are equally strong at a given time the older one will be more durable.
The joystick is an indirect input device, taking up very little space. Consisting of a small palm-sized box with a stick or shaped grip sticking up from it, the joystick is a simple device with which movements of the stick cause a corresponding movement of the screen cursor. There are two types of joystick, the absolute and the isometric. In the absolute joystick, movement is the important characteristic, since the position of the joystick in the base corresponds to the position of the cursor on the screen. In the isometric joystick, the pressure on the stick corresponds to the velocity of the cursor, and when released, the stick returns to its usual upright centred position. This type of joystick is also called the velocity-controlled joystick, for obvious reasons. The buttons are usually placed on the top of the stick, or on the front like a trigger. Joysticks are inexpensive and fairly robust, and for this reason they are often found in computer games. Another reason for their dominance of the games market is their relative familiarity to users, and their likeness to aircraft joysticks: aircraft are a favourite basis for games, leading to familiarity with the joystick that can be used for more obscure entertainment ideas.
In developing the LOGO language to teach children, Papert used the metaphor of a turtle dragging its tail in the dirt. Children could quickly identify with the real-world phenomenon and that instant familiarity gave them an understanding of how they could create pictures. Metaphors are used quite successfully to teach new concepts in terms of ones which are already understood, as we saw when looking at analogy in Chapter 1. It is no surprise that this general teaching mechanism has been successful in introducing computer novices to relatively foreign interaction
Very few will debate the value of a good metaphor for increasing the initial familiarity between user and computer application. The danger of a metaphor is usually realized after the initial honeymoon period. When word processors were first introduced, they relied heavily on the typewriter metaphor. The keyboard of a computer closely resembles that of a standard typewriter, so it seems like a good metaphor from which to start. However, the behaviour of a word processor is different from any typewriter. For example, the space key on a typewriter is passive, producing nothing on the piece of paper and just moving the guide further along the current line. For a typewriter, a space is not a character. However, for a word processor, the blank space is a character which must be inserted within a text just as any other character is inserted. So an experienced typist is not going to be able to predict correctly the behaviour of pressing the spacebar on the keyboard by appealing to her experience with a typewriter. Whereas the typewriter metaphor is beneficial for providing a preliminary understanding of a word processor, the analogy is inadequate for promoting a full understanding of how the word processor works. In fact, the metaphor gets in the way of the user understanding the computer.
New users of a system bring with them a wealth of experience across a wide number of application domains. This experience is obtained both through interaction in the real world and also through interaction with other computer systems. For a new user, the familiarity of an interactive system measures the correlation between the user's existing knowledge and the knowledge required for effective interaction. For example, when word processors were originally introduced, the analogy between
Some psychologists argue that there are intrinsic properties, or affordances, of any visual object that suggest to us how they can be manipulated. The appearance of the object stimulates a familiarity with its behaviour. For example, the shape of a door handle can suggest how it should be manipulated to open a door, and a key on a keyboard suggests to us that it can be pushed. In the design of a graphical user interface, it is implied that a soft button used in a form's interface suggests it should be pushed (though it does not suggest how it is to be pushed via the mouse). Effective use of the affordances which exist for interface objects can enhance the familiarity of the interactive system.
Another consequence of consistency having to be defined with respect to some other feature of the interaction is that many other principles can be 'reduced' to qualified instances of consistency. Hence, familiarity can be considered as consistency with respect to past real-world experience and generalizability as consistency with respect to experience with the same system or set of applications on the same platform. Because of this pervasive quality of consistency, it might be argued that consistency should be a separate category of usability principles, on the same level as learnability, flexibility and robustness. Rather than do that, we will here discuss different ways in which consistency can be manifested.
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