The goal was to structure the presentation of

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Unformatted text preview: esign of an interactive system in order to promote its usability. The goal was to structure the presentation of usability principles in such a way that the catalogue of usability principles can be extended with the increase in knowledge. The three main principles which support usability are as following: Learnability The ease with which new users can begin effective interaction and achieve maximum performance is called Learnability. Flexibility The multiplicity of ways the user and system exchange information. Robustness The level of support provided the user in determining successful achievement and assessment of goals. In following sub-sections, we will sub-divide these main categories into more specific principles, which support them. 2.5.1 Learnability Learnability relate to the features of the interactive system that allow users to understand how to use it initially and then how to attain a maximum level of performance. The specific principles which support learnability are as following: 23 Predictability Predictability of an interactive system refers that the user’s knowledge of the interaction history is sufficient to determine the results of his future interaction with this system. There are many grades to which predictability can be fulfilled. The knowledge can be limited to the presently perceivable information therefore the user don’t need to remember anything other than what is currently visible. The knowledge necessities can be increased to the limit where the user is actually forced to remember what every previous keystroke was and what every previous screen display contained information, in order to determine the consequences of the next input stroke (Dix et al.,1993). The behavior of predictability of an interactive system can be distinguished from deterministic behavior of the computer system. The majority computer systems are ultimately deterministic machines, so that set the state at any one point in time and the operation, which is to be performed at that certain time. Predictability is a user-centered notion and it is deterministic behavior from the user perspective. User must be able to take advantage of the determinism; it is not enough for the behavior of the computer system to be determined completely from its state alone (Dix et al.,1993). In order to illustrate predictability, we can take an example of a common mathematical puzzle with a sequence of three or more numbers and can ask a user what would be the next number in the sequence? The guess in this puzzle is that there is a unique function or algorithm, which produces the entire sequence of numbers and it, is up to user how he figures out. We know the function, but user just knows the results it provides from the first three calculations. The function is certainly deterministic; the test for the user is a test of its predictability given the first three numbers in the sequence. Therefore the notion of predictability deals with the user’s ability to determine the effect of operations on the system. An additional form of predictability has to do with the user’s ability to know which operation can be carried out next (Dix et al.,1993). Synthesizability Synthesizability refers to the user’s ability to assess the effect of past operations on the current situation. Primarily, predictability focuses on the user’s ability to find out the effect of future interaction; it assumes that the user has some mental model of how the system may respond. Predictability says nothing about the way the user shape a model of the system’s behavior. With the purpose of to make some sort of predictive model of the system’s behavior, it is important for the user to foresee the consequences of previous interactions in order to formulate a model of the system behavior (Dix et al.,1993). 24 Whenever an operation change occurs in the internal state, it is important that the change is observed by the user. The principle of synthesizability honesty relates to the ability of the user interface to provide an observable and informative account of any similar change. We can illustrate it by taking an example of computer file moved from one place to another place in visual and non-visual (command prompt) interface. In command prompt (system level), the user normally has to remember the destination of directory and then ask to see the contents of that directory in order to verify that the file has been moved on right place, whereas in visual interface an icon is used to move a file from one location to another location and it remains visible. In this case the visual interface is instantly honest (Dix et al., 1993). Familiarity Familiarity refers to the extent in which a user’s knowledge and experience in real word or computer-based fields can be applied while interacting with a new system. Hence, the new users of a system bring rich experience across a wise number of application domains. For a new system user, the familiarity of an interactive system measures the relation betw...
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