Data Mining Mehmed Kantardzic (good english books to read .txt) 📖

TABLE 11.2. Representing URLs as Vectors of Transaction Group Activity

The new, reduced table is the input for SOM processing. Details about the application of SOM as a clustering technique and the settings of their parameters are given in the previous chapter. We will explain only the final results and their interpretation in terms of Web-page analysis. Each URL will be mapped onto a SOM based on its similarity with other URLs in terms of user usage or, more precisely, according to users’ navigation patterns (transaction group “weights” in Table 11.2). Suppose that the SOM is a 2-D map with p × p nodes, where p × p ≥ n, then a typical result of SOM processing is given in Table 11.3. The dimensions and values in the table are not the results of any computation with values in Tables 11.1 and 11.2, but a typical illustration of the SOM’s final presentation.

TABLE 11.3. A Typical SOM Generated by the Description of URLs

The SOM organizes Web pages into similar classes based on users’ navigation patterns. The blank nodes in the table show that there are no corresponding URLs, while the numbered nodes indicate the number of URLs contained within each node (or within each class). The distance on the map indicates the similarity of the Web pages measured by the user-navigation patterns. For example, the number 54 in the last row shows that 54 Web pages are grouped in the same class because they have been accessed by similar types of people, as indicated by their transaction patterns. Similarity here is measured not by similarity of content but by similarity of usage. Therefore, the organization of the Web documents in this graphical representation is based solely on the users’ navigation behavior.

What are the possible applications of the LOGSOM methodology? The ability to identify which Web pages are being accessed by a company’s potential customers gives the company information to make improved decisions. If one Web page within a node successfully refers clients to the desired information or desired page, the other pages in the same node are likely to be successful as well. Instead of subjectively deciding where to place an Internet advertisement, the company can now decide objectively, supported directly by the user-navigation patterns.

11.4 MINING PATH–TRAVERSAL PATTERNS

Before improving a company’s Web site, we need a way of evaluating its current usage. Ideally, we would like to evaluate a site based on the data automatically recorded on it. Each site is electronically administered by a Web server, which logs all activities that take place in it in a file called a Web-server log. All traces left by the Web users are stored in this log. Therefore, from these log files we can extract information that indirectly reflects the site’s quality by applying data-mining techniques. We can mine data to optimize the performance of a Web server, to discover which products are being purchased together, or to identify whether the site is being used as expected. The concrete specification of the problem guides us through different data-mining techniques applied to the same Web-server log.

While the LOGSOM methodology is concentrated on similarity of Web pages, other techniques emphasize the similarity of a user’s paths through the Web. Capturing user-access patterns in a Web environment is referred to as mining path–traversal patterns. It represents an additional class of data-mining techniques, which is showing great promise. Note that because users travel along information paths to search for the desired information, some objects or documents are visited because of their location rather than their content. This feature of the traversal pattern unavoidably increases the difficulty of extracting meaningful information from a sequence of traversal data, and explains the reason why current Web-usage analyses are mainly able to provide statistical information for traveling points, but not for traveling paths. However, as these information-providing services become increasingly popular, there is a growing demand for capturing user-traveling behavior to improve the quality of such services.

We first focus on the theory behind the navigational patterns of users in the Web. It is necessary to formalize known facts about navigation: that not all pages across a path are of equal importance, and that users tend to revisit pages previously accessed. To achieve a data-mining task, we define a navigation pattern in the Web as a generalized notion of a sequence, the materialization of which is the directed-acyclic graph. A sequence is an ordered list of items, in our case Web pages, ordered by time of access. The log file L is a multiset of recorded sequences. It is not a simple set, because a sequence may appear more than once.

When we want to observe sequence s as a concatenation of the consecutive subsequences x and y, we use the notation:

The function length(s) returns the number of elements in the sequence s. The function prefix(s, i) returns the subsequence composed of the first i elements of s. If s’ = prefix(s,i), we say that s’ is a prefix of s and is denoted as s’ ≤ s. Analysis of log files shows that Web users tend to move backwards and revisit pages with a high frequency. Therefore, a log file may contain duplicates. Such revisits may be part of a guided tour or may indicate disorientation. In the first case, their existence is precious as information and should be retained. To model cycles in a sequence, we label each element of the sequence with its occurrence number within the sequence, thus distinguishing between the first, second, third, and so on occurrence of the same page.

Moreover, some sequences may have common prefixes. If we merge all common prefixes together, we transform parts of the log file into a tree structure, each node of which is annotated with the number of sequences having the same prefix up to and including this node. The tree contains the same information as the initial log file. Hence, when we look for