This paper deals with one aspect of the index quality of search engines: index freshness. The purpose is to analyse the update strategies of the major Web search engines Google, Yahoo, and MSN/Live.com. We conducted a test of the updates of 40 daily updated pages and 30 irregularly updated pages, respectively. We used data from a time span of six weeks in the years 2005, 2006, and 2007. We found that the best search engine in terms of up-to-dateness changes over the years and that none of the engines has an ideal solution for index freshness. Frequency distributions for the pages' ages are skewed, which means that search engines do differentiate between often-and seldom-updated pages. This is confirmed by the difference between the average ages of daily updated pages and our control group of pages. Indexing patterns are often irregular, and there seems to be no clear policy regarding when to revisit Web pages. A major problem identified in our research is the delay in making crawled pages available for searching, which differs from one engine to another. • Search engine usability: This gives a feedback of user behaviour and is evaluated by user surveys, laboratory tests, or transaction log analyses. The present study solely deals with a part of the index quality section. We believe that index freshness is an important part of quality measurement. Search engines should provide up-to-date information. We hope that the results from this study will be useful for being part of our overall search engine quality analysis. Up-to-dateness with search engines derives its importance from several factors. First, there is the sheer size of the Web (see e.g, [2, 7-9] and its ever-changing contents. New pages are built, old pages are deleted, and links are changed, all at a high rate. But because of the growth of the Web, the number of old pages that no longer change has also increased significantly. Search engines have to find ways to show to the user pages that meet his or her up-to-dateness criteria. In some cases, older pages may be helpful, but in the majority of cases, one would assume that a user prefers current ones. A study by Ntoulas, Cho, and Olston [10] found that a large number of Web pages are changing on a regular basis. Estimating the results of the study extrapolated over the entire Web, the authors find that there are about 320 million new pages every week. About 20 percent of the Web pages of today will disappear within a year. About 50 percent of all contents will be changed within the same period. The link structure will change even faster: About 80 percent of all links will have changed or will be new within a year. Although the absolute values may be out of date now, the results show how important it is for the search engines to keep their databases up to date. Huge and fast changes in the Web's contents are also reported in [11] [12] [13] [14] . Bar-Ilan [15] studies the reasons for differing search engine results pages. Among others, she lists the following reasons directly related to up-to-dateness: • Some of the search engines have several query engines or databases • The index is partitioned • When the crawler refreshes its database, some of the previously visited pages may be unreachable due to communication or server failure • Fluctuations may be due to changes in the indexing policy of the search engines or in the size of the databases. These are important factors that could explain inconsistencies in the results, as we will report below. Ke, Deng, Ng, and Lee [16] give a good overview of the problems for search engines resulting from Web dynamics. Crawling and indexing problems resulting from Web dynamics from a commercial search engine's point of view can be found in Risvik and Michelsen [17]. Arguably every search engine user has experienced 404 errors with search engines (i.e., the page found by the engine links to a page that is no longer available). Newer studies [18, 19] show that the number of these errors is relatively low (for the top 20 results, between 2.2 and 6.5 percent and for the top 10 results, between 2.0 and 8.9 percent, depending on search engine), but even so, they are a major nuisance [20], pp. 179-180 [21] that results from unsuitable index freshness. But how should search engines keep their indices up to date? It is quite clear that no search engine is able to update its complete index on a daily basis. This has economic as well as technical reasons, which we will discuss further in the next section. It seems to be agreed among practitioners that search engines should index all pages in their indices in a cycle of one month. In our previous research, we were able to show that this does not hold true for the major search engines Google and Yahoo, even for pages whose contents are updated on a daily basis [5] .But the question remains: Should search engines stick to such an update schedule, or can older pages be kept unindexed for a longer time? We will try to find an answer to this question, too. This paper is organised as follows: First, we will give a concise review on the literature on search engine freshness, then we will state our objectives, and after that we will describe our methods. The emphasis is on the results of the current study in comparison to our previous study [5] . In the final section, we will draw conclusions and show areas for further research.
Finally, we want to highlight the article REF , where Lewandowski performed a study over 3 years analyzing the update strategies of different search engines on a set of web pages.
A three-year study on the freshness of web search engine databases
J. Inf. Sci.
J. Inf. Sci.
Computer Science