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Article Excerpt
INTRODUCTION
The number of children possessing cellular phones has increased remarkably within recent years. At least 45% of Germany's 12- to 13-year-olds own such a technical device (CHIP Online, 2003).
The numerous cellular phone models of various brands on the market differ in menu structure as well as number and functionality of keys for navigating through the hierarchical menu. Commonly agreed rules as to which arrangement of functions and keys lead to the best usability have not been defined. Perhaps that is why manufacturers generally allocate few resources to investigating usability, issues. Some emphasize "ease and joy of use for complex systems" (e.g., Siemens, as reported in Stanney & Salvendy, 2000), but this may be more of a marketing tool than a focus of research. Even researchers argue that modern phones do not differ much, apart from their shape and color (e.g., Harper, 2001). and the industry may assume that the prevailing small differences in usability' of the phones will become less important in the future because of today's children's ability to handle technological systems easily. Compared with adults, who grew up in a less technological environment, kids are supposed to understand the mode of operation of these devices much faster by virtue of their early contact with interactive technology (e.g., computers, video games). Additionally, children's fascination for explorative and inquisitive activities is well known. However, little to no research has been conducted to evaluate children's abilities using technical products, and hence these assumptions have no empirical foundation, even if they seem to be common sense.
Good usability of cellular phones is crucial: Telecommunication providers are investing huge sums of money in new services and technologies (e.g., UMTS, or Universal Mobile Telecommunications System, and WAP, or Wireless Application Protocol), which will be used only if the phone enables quick and easy access to the desired information. Children are certainly a customer group focused on by the industry.
Research Issues and Experimental Logic
The current level of knowledge about children's purposeful interaction with technical devices is very low. Thus it is important to explore how differences in complexity affect the usability of cellular phones for young novice users as well as which specific difficulties they experience.
The factors that influence the complexity of a cellular phone, and thus the difficulties users have in learning how to efficiently use it, have not been systematically analyzed. In the literature, approaches can be found that define the complexity of devices by decomposing users' behavior on the keystroke level, quantifying the number of operations the user has to carry out. In Kieras and Polson (1985), for example, the number of production rules (condition-action pairs in the form "if" [condition] "then" [action]) the user has to learn to operate a system is of central importance. Applying this approach to cellular phones, it may be assumed that the more production rules that have to be learned and memorized by the user to carry out certain tasks, the higher the system's complexity. In order to solve tasks on a cellular phone, one has to know which menu entries have to be selected and which keys need to be pressed to select them. Accordingly, for each event in the display a production rule of the form "if-then" needs be learned; what is seen in the display by the user corresponds to the "if" (condition), and the keystroke required (e.g., to select a function or to scroll to the next item) corresponds to the "then" (action).
To fulfill the demand for ecological validity, two existing models of different brands, the Nokia 3210 and the Siemens C35i, were selected for this study. These phones have comparable functionality but differ in the number of production rules required to carry out frequently used functions.
Table 1 shows the number of production rules that have to be learned to solve four common tasks (calling someone using the phone directory, sending a short text message, hiding their own number, and editing an entry in the phone book) on these two mobile phones.
To carry out these four functions using the Siemens C35i a total of 45 production rules need to be learned, whereas for the Nokia 3210 only 36 production rules are required. As the difficulty of each rule cannot be predicted with the applied theoretical approach, all rules are assumed to be of the same complexity and therefore equally easy to learn. Taking the "easier to use" phone always as baseline, and assuming a linear relationship between number of production rules and time needed to complete tasks, leads to the conclusion that it should take in total approximately 25% longer to solve the tasks with the Siemens phone than with the Nokia. Regarding the single tasks, it is expected (according to the number of production rules) that users in the Nokia group will clearly outperform Siemens users in sending a short message (10 vs. 14 production rules) and hiding their own number (12 vs. 19 production rules), whereas Siemens users should show slightly superior performance when calling somebody (3 vs. 4 production rules) and when editing a number in the phone directory (9 vs. 10 production rules). As the adequacy of a user's task representation presumably varies from task to task (the notion of how to call somebody should he more adequate than how to hide their own number when calling, given that the first is frequently used on fixed-line phones), comparisons between only the two phones regarding each task will be undertaken, not between different tasks.
It is unclear whether the described formalism is adequate to predict...
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