ساختار ناوبری وب در گوشی های تلفن همراه: عمق / پهنا مسأله تجارت کردن

One can browse the web with a variety of devices, including hand-held devices such as the cellular phone. The small screen of those devices poses some serious usability issues, one of which is the appropriate hierarchy depth of the web site. In this study, we empirically examined whether a broad navigation structure, which was found to be superior in regular screen-size platforms, also has an advantage for a small-screen device such as the cellular phone where it may require more movements and scrolling between screens of the same hierarchical level. Navigation times and success rates were measured for two search tasks in a mock web site that was built in two versions: one with a broad navigation structure and the other with a deep structure. Both structures were tested with cellular phone emulation and a standard desktop personal computer (PC). Results indicate that performance was better with the broad navigation structure for both the cellular phone and the PC. In addition, performance was better with the PC as compared to the cellular phone, and this difference was pronounced in the broad structure. The results are discussed in terms of the impact of device-independent characteristics of the hierarchy depth along with the theoretical account of increased working memory load, confusion and disorientation associated more with deep structures.

With the advent of today's technology, web browsing has become an activity that can be performed with a variety of platforms and devices such as the desktop personal computer (PC), interactive TV, cellular telephones, Personal Digital Assistants and more. Platforms such as the cellular phone and Personal Digital Assistants have unique operational characteristics that directly influence their web browsing usability and thus presenting developers with unique design challenges. The major influencing characteristics are the small physical display and the mobile context of usage. In this paper we are focusing on the impact of the small display on critical aspects of the navigation structure in web sites. The root of the usability problem is the desire to let users of small screen devices have access to the same information that is presented in the fixed, regular-size screen platforms such as the desktop PC in addition to letting the user perform similar tasks with this information (Buchanan et al., 2001; Ericsson et al., 2001). Typical approaches to deal with this requirement are to either deliver the information as is or change the data structure, reformat and adapt it to the small screen (Watters et al., 2003). Various ways of adaptation were developed, the primary one being re-formatting page layout. Reformatting guidelines were published for Personal Digital Assistants (e.g., AvantGo; Palm OS) and for cellular phones, primarily Wireless Application Protocol phones (e.g., WAP Forum, 2001; Sprint Spectrum, 2002; Nokia Corporation, 2002; Openwave, 2003). One line of investigation that is related to the content reformatting approach was to examine the ability to read information from small screens (e.g., Duchnicky and Kolers, 1983; Reseil and Shneiderman, 1987; Jones et al., 1999). In general it was found that smaller display size with shorter text lines degraded visual search and reading performance. Another line of development and research looked at the feasibility and efficacy of various dynamic content adaptations to the small screen. These included the use of Rapid Serial Visual Presentation to solve the space–time tradeoff of small screen presentations (e.g., de Bruijn et al., 2002; Ford et al., 1997), the use of transparent navigation mechanisms (Kamba et al., 1996), the use thumbnailing of web pages to enhance search (e.g., Wobbrock et al., 2002), and real-time restructuring of web pages to fit the small screen (e.g., Keranen and Plomp, 2002; Buyukkokten et al., 2001; Chen et al., 2003). Another common approach to adapt the content to small displays is to restructure the information and divide it into smaller displayable chunks. In general it was found that such an approach was associated with more navigation steps required in the small screen device, such as backward and forward (e.g., Dillon et al., 1999), longer web search and browsing history usage (de Bruijn et al., 2002), and longer browsing paths (Jones et al., 1999). Taken together, the approach of information re-structuring in small-display devices was shown to influence navigation in the web site. Regardless of the platform, information structure or architecture is critical to web site navigation (Brinck et al., 2002; Neilsen, 2000). There are two primary factors that characterize web site structure or the navigation structure: the number of items per page and the number of levels in the site. When there are few items in a page (i.e., small chunks), it creates many levels and the structure is considered to be deep. When there are many items in a page (large chunks), the structure is considered to be wide or broad since there is no need for many levels. There is an inherent tradeoff between the depth and the breadth in the navigation structure. Various factors such as visual search time, motor response time, limitations of human working memory and others are associated with the impact of this tradeoff on navigation performance. While this tradeoff was studied extensively in regular-size displays, very little attention was directed to examining it in small-screen platforms. The research that deals with the breadth-depth tradeoff in the desktop PC or other fixed, regular screen-size platforms, looked primarily at menu structures and showed a consistent advantage of broad menu structures with respect to performance times and accuracy rates (Allen, 1983; Hagelbarger and Thompson, 1983; Kiger, 1984; Snowberry et al., 1983; Seppala and Salvendy, 1985; Tullis, 1985). In addition, when the item that the participant had to find was deep in the tree, the average time in each level was longer (Allen, 1983; Hagelbarger and Thompson, 1983). Less research was done with respect to the impact of menu structure on forms filling interaction. Norman et al. (2001) showed that when the navigation is linear (there is no need to jump back and forth in the form), participants preferred scrolling down the page or using the “next page/item” links rather than navigate by choosing the direct links. This indicated that for this kind of form filling, a broad menu is better than a deep one. No other research was found to give further support to this conclusion. In contrast, the considerations for choosing between a broad vs. a deep menu or navigation structure can be different and fuzzier for platforms with a small physical screen such as the Personal Digital Assistant and cellular phone. Tang (2001) and Ziefle (2002) present the problems in broad menus for a cellular phone. The problems are caused primarily by the inability to display all the items of the same level (that are on the same page) in one screen as it is on the PC, and as a result they need to be divided and displayed in several screens. As a consequence, users have to gather information from each of these screens as they navigate in the same level, in addition to gathering information from other levels. Thus, a broader menu will not necessarily alleviate the load on the working memory even though it has fewer levels as compared to navigating through many levels in a deep menu structure (Ziefle, 2002). In addition to this problem, Tang (2001) indicated that navigation in cellular phones requires the user to execute motor actions such as pressing keys in order to scroll between screens that belong to the same level (the same page). Consequently, there maybe more user actions required in a broad structure in a small-screen device and these actions may take more time as compared to a deep menu. Very little research looked at the tradeoffs inherent in various web site structures in terms of the influence on navigation performance in a small-screen device. Tang (2001) compared between a broad and a deep structure and showed that it took more time to execute the tasks in the deep menu. Kaikkonen and Roto (2003) compared a flat structure (one level) to a deep structure (two levels) and showed that task execution times were shorter for the flat structure. In addition, Chen et al. (2003) showed that by dynamically splitting pages into two levels, user's browsing experience was improved. However, these studies did not examine the typical deeper navigation structures, 3–6 levels deep, which are often encountered in web sites. In addition, no rigorous empirical research was done to examine the impact of navigation structure on form filling interaction with the items that are searched for in a cellular phone. The little research that addressed the issue of navigation structures in small-screen devices is not as conclusive as the research that addressed this issue in regular screen-size. Specifically, the question remains whether a broad navigation structure in a cellular phone is better than a deep structure, as was shown for the desktop and other platforms with a larger screen. Conversely, a broad structure is not necessarily better because of the small physical screen and the need to display the information of the same page in several screens, which would require more scrolling actions. This question is significant from both theoretical and practical aspects. Theoretically, the question is whether the influence of hierarchy depth in navigation structures is device dependent or independent. If the influence is device dependent we should not expect an advantage to the broad structure in the small-screen device such as a cellular phone (Tang, 2001; Ziefle, 2002). This can be expected because a broad structure in the small-screen device may entail more user actions than the deep structure as was explained above. However, if the influence is device independent and is due to other factors such as human memory limits, then we should expect to see an advantage to the broad structure in the small-screen device similar to what is observed with the regular screen platforms. Practically, there is still a need to formulate clear recommendations and guidelines for the design and information architecture of web sites for small-screen devices. The objective of the study reported here was to empirically examine the breadth–depth tradeoff in the navigation structure of a web site in a cellular phone. An identical web site in a desktop platform was used as a control condition to replicate the advantage of the broad structure reported in the literature. The research was conducted using a mock web site of movie listings and movie ticket booking. For each of the platforms, cellular phone and desktop, two user interfaces were designed for browsing the site. The user interfaces were different in their navigation structures, but identical in every other aspect. One interface was broader and less deep (“broad” from hereon)—it consisted of many items per page along with fewer levels. The other interface was deeper (“deep” from hereon)—it consisted of few items per page along with five levels. The experimental tasks were to find a certain movie and book tickets to that movie by filling in certain details. The sequence of steps and the time to perform every step were recorded. Another important aspect of the measures in this study was the characterization of various navigation patterns and errors that can be related to the research question.

The findings of this study show similarity between the desktop PC and cellular phone in terms of the impact of hierarchy depth of the navigation structure on performance. There was an advantage to browsing in a broad structure as compared to a deep structure for both platforms. In addition, web browsing in the desktop PC was performed faster and better than in the cellular phone. The following discussion is divided into three parts: a summary of the findings, followed by a discussion of the theoretical implications of those findings, and finally the practical implication and future research agenda are presented. 4.1. Summary of the findings 4.1.1. Overall task success rates The success rate in the different conditions can be viewed as a measure of effectiveness. According to this measure, task effectiveness was high: success rates were in the range of 80–100%, and were similar across platforms, navigation structures and the two tasks. In addition, there was a higher success rate for task 1 with the PC as compared to the cellular phone. 4.1.2. Navigation durations The navigation time in the desktop platform was shorter than the navigation time in the cellular phone platform. This finding is in line with findings reported by Kim and Albers (2001). The navigation time in the broad structure was shorter than the navigation time in the deep structure. The advantage of the broad structure was found both for the PC platform and for the cellular phone platform. The differences between the platforms were more pronounced with the broad navigation structure, particularly with task 2. Finally, the navigation time in task 1 was shorter than the navigation time in task 2. 4.1.3. Sub-optimal navigation sequences and errors The various sub-optimal navigation sequences and errors that were made during navigation were classified and analysed in order to characterize and examine more closely the advantage of the broad navigation structure in cellular phone. For task 1 in the broad structure, there were less premature exits and fewer attempts to book tickets for the wrong movie. For task 2 there was also an advantage to the broad structure in terms of having less premature exits. In addition, there was an advantage of the broad structure in terms of less multiple viewing of several items. 4.1.4. Ticket booking durations There were no differences between the different platforms and the different navigation structures in the time it took to complete ticket booking. The lack of difference in performance durations between the platforms and between the two navigation structures, for each platform, is somewhat surprising. It is surprising because one would expect the interaction type of form filling to be more familiar with the PC and thus be performed faster than with the cellular phone platform. In addition, faster performance would be expected with a broad structure of form filling because all the details are in a single screen as opposed to a deep structure, where the details are divided into several screens (also see Norman et al., 2001). It is possible that navigation structures differing in the depth of the hierarchy have their primary impact on the navigation part of the task and less in the interactive part, such as ticket booking in this study. 4.1.5. Ticket booking errors There was an advantage to the broad structure, for both tasks, in terms of less pressing the “SELECT” key after entering the data. This error can be characterized in two ways. One is that using the “SELECT” key implies confusion with the navigation keys such as the UP, DOWN and EXIT keys. The other is that using the “SELECT” key implies erroneous understanding that there is a need to confirm every item that is filled in instead of the requirement to confirm only at the end of the interaction. The increased occurrence of this error in the deep structure is discussed below. 4.2. Theoretical implications The findings for the desktop platform in this study are in line with many previous reports indicating that navigation performance in a broad structure is better than navigation in a deep structure (e.g., Norman, 1991; Snowberry et al., 1983; Kiger, 1984; Tullis, 1985; Seppala and Salvendy, 1985). The important empirical finding in this study is that similar navigation structures implemented in a cellular phone with a small screen also produced performance advantages with the broad structure. The question is: what can account for this performance similarity between platforms that are very different operationally? On the one hand, the findings here may imply that the impact of hierarchy depth in navigation structures is due to device-independent factors. In previous research with regular-size displays, such as the desktop PC and telecommunication monitors, increased load on user's working memory was characterized as being associated mainly with the deep navigation structure and thus accounting for the poorer performance (e.g., Seppala and Salvendy, 1985; Tullis, 1985; Norman, 1991). In deep menu structures, the user has to make more choices by having to select between various options in every level, and having to go through more levels. Every such choice increases visual search time, decision time and reaction time (Seppala and Salvendy, 1985; Norman, 1991). Thus, the determining device-independent factor affecting performance is having more actions and increased working memory load associated with navigating through more levels in deep hierarchies. On the other hand, the findings here cannot be accounted for by simply assuming that there are fewer user actions in broad structure as compared to a deep structure. The broad structure in a small-screen device does not necessarily require less user actions in comparison to a deep structure. A given number of items per page in the small-screen device can be distributed, in the broad structure, on many more screens, and consequently, there can be many selection actions—moving between screens in the same level and moving between levels (e.g., Tang, 2001). In this study, for example, the minimal number of clicks required to complete task 1 in the broad structure of the cellular phone was more that the minimal number of clicks required to complete it in the deep structure. From this perspective, because of the small screen of the cellular phone, it is possible that increased working memory load will also be associated with a broad structure which demands the user to move between several screens in the same level. In other words, smaller number of actions or less working memory load alone may not be sufficient to account for the consistent advantage of the broad structure in the cellular phone. Another perspective that can further account for the broad structure advantage is the increased confusion and disorientation associated with deep navigation hierarchies. In some of the decision points in the hierarchy, the terms for describing the categories could be vague and ambiguous, and the user may have trouble identifying which category should be selected in order to reach a certain item. Consequently, there is more uncertainty and confusion in deep menus about the location of the desired items (Tullis, 1985; Norman, 1991). The details about the navigation structure, the items that need to be stored in the working memory, and the additional load due to the characteristics of the deep hierarchy, could make it difficult to store or retrieve those details. Consequently, the users may become disoriented: they may forget where they came from, which places they visited already, and where to go next. Examination of the characteristics of sub-optimal navigation sequences and errors committed in this study with the cellular phone platform imply more confusion and disorientation in the deep structure as compared to the broad structure. Specifically, behaviors such as premature exit, multiple viewing of several items and trying to book the wrong movie reflect confusion and disorientation. Moreover, the error of unnecessarily pressing on the “SELECT” key may reflect confusion between navigation (moving from one screen to another) and specific data-related action (confirmation). In contrast, navigation in the broad structure, even if it means more scrolling between screens that are on the same hierarchical level, would be less associated with disorientation. This is because the user, while scrolling between physical screens, may still maintain position awareness such as “I know where I am—I am still in the same level”. The increased amount of scrolling performed with the broad structure in cellular phones as opposed to the PC can account for the more pronounced differences between the platforms in the broad structure. Another factor that needs to be considered is the difference between the two tasks. Because of the greater decision complexity in task 2, the participant needed to gather more information before reaching the ticket booking part. This in turn may have increased the load on the working memory. This additional load degraded performance in task 2 in general. In addition, the increased memory load due to the task complexity and due to the deep structure made the task almost equally difficult in the deep structure in both platforms. 4.3. Practical implications and research agenda Generalization of the findings of this study should be qualified. The main limit of the external validity of this study is the use of an on-screen emulation of the cellular phone. Such setup excludes some important factors that are associated with the mobile context of use typical of the cellular phone. The mobile context probably has an impact on user performance, however much can be investigated in the laboratory context. In particular, this study examined a basic conceptual issue of the web site structure and its influence on navigation performance. Such issues can be studied in simulators, emulators or even “paper prototypes”, since most of the determining performance factors are not motor actions but rather cognitive—visual search, short-term memory, decision making, orientation and position awareness, comprehension and mental models (e.g., Seppala and Salvendy, 1985; Tullis, 1985; Norman, 1991). The general recommendation for web site architecture is to limit the depth of the hierarchy rather than its breadth (e.g., Brinck et al., 2002; Larson and Czerwinski, 1998). Those recommendations referred primarily to web sites displayed on regular screen-size platforms such as the desktop PC. Based on the findings of this study, taking into consideration its external validity limit, and in line with very few other studies (Chen et al., 2003; Kaikkonen and Roto, 2003), a similar recommendation can be formulated for small-screen devices such as the cellular phone. Browsing performance with a broad structure in cellular phones would still be inferior compared with regular screen-size platforms, but it would improve performance relative to breaking the content of the site into smaller chunks. It should be emphasized that designing a broader navigation structure for web sites displayed on small screen devices does not solve problems such as reading from small screens and the need to perform more scrolling actions. A line of research and development that should be pursued is the examination of various content adaptation techniques combined with broad navigation structures.