Some secrets about Google...March 31, 2005, the US Patent & Trademark Office made public a patent registered by the managers of Google on December 31, 2003. This patent aims at protecting some of the techniques and technologies used by Google.
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Incidentally, this patent DESCRIBES some of the techniques which are, can or could be used by Google to calculate the classification of the pages presented in its results of research. The totality of the techniques described here is not necessarily used by Google. This informations should not thus be regarded as a description of the operation of Google. It is ONLY about one patent filed by Google with an aim at protecting its ideas.
However, it is the first time that such a significant document is accessible to the public and its contents are fairly interesting because it finally enables us to have a broad outline of the Google's philosophy.
The original can be consulted at: US Patent & Trademark Office
Here a short summary of this document which tries to be more readable than the original. Some sentences have been deleted or truncated to simplify the whole text.
Inception date and "age" of a document
- A document's inception date may be used to generate (or alter) a score associated with that document. The inception date of a document may be determined from the date that search engine (...) first discovers a link to the document.
- It may be assumed that a document with a fairly recent inception date will not have a significant number of links from other documents (i.e., back links). For existing link-based scoring techniques that score based on the number of links to/from a document, this recent document may be scored lower than an older document that has a larger number of links (e.g., back links). When the inception date of the documents are considered, however, the scores of the documents may be modified (either positively or negatively) based on the documents' inception dates.
Consider the example of a document with an inception date of yesterday that is referenced by 10 back links. This document may be scored higher by search engine than a document with an inception date of 10 years ago that is referenced by 100 back links because the rate of link growth for the former is relatively higher than the latter. While a spiky rate of growth in the number of back links may be a factor used by search engine to score documents, it may also signal an attempt to spam search engine. Accordingly, in this situation, search engine may actually lower the score of a document(s) to reduce the effect of spamming.
- Thus, according to an implementation consistent with the principles of the invention, search engine may use the inception date of a document to determine a rate at which links to the document are created (e.g., as an average per unit time based on the number of links created since the inception date or some window in that period). This rate can then be used to score the document, for example, giving more weight to documents to which links are generated more often.
For some queries, older documents may be more favorable than newer ones. As a result, it may be beneficial to adjust the score of a document based on the difference (in age) from the average age of the result set. In other words, search engine may determine the age of each of the documents in a result set (e.g., using their inception dates), determine the average age of the documents, and modify the scores of the documents (either positively or negatively) based on a difference between the documents' age and the average age.
- Information relating to a manner in which a document's content changes over time may be used to generate (or alter) a score associated with that document. For example, a document whose content is edited often may be scored differently than a document whose content remains static over time. Also, a document having a relatively large amount of its content updated over time might be scored differently than a document having a relatively small amount of its content updated over time.
- Update Amount may be used to influence the score assigned to a document. For example, the rate of change in a current time period can be compared to the rate of change in another (e.g., previous) time period to determine whether there is an acceleration or deceleration trend. Documents for which there is an increase in the rate of change might be scored higher than those documents for which there is a steady rate of change, even if that rate of change is relatively high.
For some queries, documents with content that has not recently changed may be more favorable than documents with content that has recently changed. As a result, it may be beneficial to adjust the score of a document based on the difference from the average date-of-change of the result set. In other words, search engine may determine a date when the content of each of the documents in a result set last changed, determine the average date of change for the documents, and modify the scores of the documents (either positively or negatively) based on a difference between the documents' date-of-change and the average date-of-change.
One or more query-based factors may be used to generate (or alter) a score associated with a document. For example, one query-based factor may relate to the extent to which a document is selected over time when the document is included in a set of search results. In this case, search engine might score documents selected relatively more often/increasingly by users higher than other documents.
- Another query-based factor may relate to the occurrence of certain search terms appearing in queries over time. A particular set of search terms may increasingly appear in queries over a period of time. For example, terms relating to a "hot" topic that is gaining/has gained popularity or a breaking news event would conceivably appear frequently over a period of time. In this case, search engine may score documents associated with these search terms (or queries) higher than documents not associated with these terms.
- Another query-based factor may relate to queries that remain relatively constant over time but lead to results that change over time. For example, a query relating to "world series champion" leads to search results that change over time (e.g., documents relating to a particular team dominate search results in a given year or time of year). This change can be monitored and used to score documents accordingly.
- In some situations, a stale document may be considered more favorable than more recent documents. As a result, search engine may consider the extent to which a document is selected over time when generating a score for the document. For example, if for a given query, users over time tend to select a lower ranked, relatively stale, document over a higher ranked, relatively recent document, this may be used by search engine as an indication to adjust a score of the stale document.
- Yet another query-based factor may relate to the extent to which a document appears in results for different queries. In other words, the entropy of queries for one or more documents may be monitored and used as a basis for scoring. For example, if a particular document appears as a hit for a discordant set of queries, this may (though not necessarily) be considered a signal that the document is spam, in which case search engine may score the document relatively lower.
- The "appearance date" of a link may be the first date that search engine finds the link or the date of the document that contains the link (e.g., the date that the document was found with the link or the date that it was last updated). The "disappearance date" of a link may be the first date that the document containing the link either dropped the link or disappeared itself.
Using this dates as references, search engine may then monitor the time-varying behavior of links to the document, such as when links appear or disappear, the rate at which links appear or disappear over time, how many links appear or disappear during a given time period, whether there is trend toward appearance of new links versus disappearance of existing links to the document, etc.
- Using the time-varying behavior of links to (and/or from) a document, search engine may score the document accordingly. For example, a downward trend in the number or rate of new links (e.g., based on a comparison of the number or rate of new links in a recent time period versus an older time period) over time could signal to search engine that a document is stale, in which case search engine may decrease the document's score. Conversely, an upward trend may signal a "fresh" document (e.g., a document whose content is fresh--recently created or updated) that might be considered more relevant, depending on the particular situation and implementation.
- By analyzing the change in the number or rate of increase/decrease of back links to a document (or page) over time, search engine may derive a valuable signal of how fresh the document is. For example, if such analysis is reflected by a curve that is dropping off, this may signal that the document may be stale (e.g., no longer updated, diminished in importance, superceded by another document, etc.).
- Models may be built that predict if a particular distribution of link dates signifies a particular type of site (e.g., a site that is no longer updated, increasing or decreasing in popularity, superceded, etc.).
- Each link may be weighted by a function that increases with the freshness of the link. The date of appearance/change of the document containing a link may be a better indicator of the freshness of the link based on the theory that a good link may go unchanged when a document gets updated if it is still relevant and good. In order to not update every link's freshness from a minor edit of a tiny unrelated part of a document, each updated document may be tested for significant changes (e.g., changes to a large portion of the document or changes to many different portions of the document) and a link's freshness may be updated (or not updated) accordingly.
- Links may be weighted based on how much the documents containing the links are trusted (e.g., government documents can be given high trust). Links may also, or alternatively, be weighted based on how authoritative the documents containing the links are (e.g., authoritative documents may be determined in a manner similar to that described in U.S. Pat. No. 6,285,999). Links may also, or alternatively, be weighted based on the freshness of the documents containing the links using some other features to establish freshness (e.g., a document that is updated frequently (e.g., the Yahoo home page) suddenly drops a link to a document).
- Search engine may raise or lower the score of a document to which there are links as a function of the sum of the weights of the links pointing to it. This technique may be employed recursively. For example, assume that a document S is 2 years olds. Document S may be considered fresh if n % of the links to S are fresh or if the documents containing forward links to S are considered fresh. The latter can be checked by using the creation date of the document and applying this technique recursively.
- The dates that links appear can also be used to detect "spam," where owners of documents or their colleagues create links to their own document for the purpose of boosting the score assigned by a search engine. A typical, "legitimate" document attracts back links slowly. A large spike in the quantity of back links may signal a topical phenomenon (e.g., the CDC web site may develop many links quickly after an outbreak, such as SARS), or signal attempts to spam a search engine (to obtain a higher ranking and, thus, better placement in search results) by exchanging links, purchasing links, or gaining links from documents without editorial discretion on making links. Examples of documents that give links without editorial discretion include guest books, referrer logs, and "free for all" pages that let anyone add a link to a document.
The disappearance of many links can mean that the document to which these links point is stale (e.g., no longer being updated or has been superseded by another document). For example, search engine may monitor the date at which one or more links to a document disappear, the number of links that disappear in a given window of time, or some other time-varying decrease in the number of links (or links/updates to the documents containing such links) to a document to identify documents that may be considered stale. Once a document has been determined to be stale, the links contained in that document may be discounted or ignored by search engine when determining scores for documents pointed to by the links.
>- Information relating to a manner in which anchor text changes over time may be used to generate (or alter) a score associated with a document. For example, changes over time in anchor text associated with links to a document may be used as an indication that there has been an update or even a change of focus in the document.
- Alternatively, if the content of a document changes such that it differs significantly from the anchor text associated with its back links, then the domain associated with the document may have changed significantly (completely) from a previous incarnation. This may occur when a domain expires and a different party purchases the domain. Because anchor text is often considered to be part of the document to which its associated link points, the domain may show up in search results for queries that are no longer on topic. This is an undesirable result. One way to address this problem is to estimate the date that a domain changed its focus. This may be done by determining a date when the text of a document changes significantly or when the text of the anchor text changes significantly. All links and/or anchor text prior to that date may then be ignored or discounted.
The freshness of anchor text may also be used as a factor in scoring documents. The freshness of an anchor text may be determined, for example, by the date of appearance/change of the anchor text, the date of appearance/change of the link associated with the anchor text, and/or the date of appearance/change of the document to which the associated link points. The date of appearance/change of the document pointed to by the link may be a good indicator of the freshness of the anchor text based on the theory that good anchor text may go unchanged when a document gets updated if it is still relevant and good. In order to not update an anchor text's freshness from a minor edit of a tiny unrelated part of a document, each updated document may be tested for significant changes (e.g., changes to a large portion of the document or changes to many different portions of the document) and an anchor text's freshness may be updated (or not updated) accordingly.
Information relating to traffic associated with a document over time may be used to generate (or alter) a score associated with the document. For example, search engine may monitor the time-varying characteristics of traffic to, or other "use" of, a document by one or more users. A large reduction in traffic may indicate that a document may be stale (e.g., no longer be updated or may be superseded by another document).
- Search engine may compare the average traffic for a document over the last j days (e.g., where j=30) to the average traffic during the month where the document received the most traffic, optionally adjusted for seasonal changes, or during the last k days (e.g., where k=365). Optionally, search engine may identify repeating traffic patterns or perhaps a change in traffic patterns over time. It may be discovered that there are periods when a document is more or less popular (i.e., has more or less traffic), such as during the summer months, on weekends, or during some other seasonal time period. By identifying repeating traffic patterns or changes in traffic patterns, search engine may appropriately adjust its scoring of the document during and outside of these periods.
- Additionally, or alternatively, search engine may monitor time-varying characteristics relating to "advertising traffic" for a particular document. For example, search engine may monitor one or a combination of the following factors:
(1) the extent to and rate at which advertisements are presented or updated by a given document over time;
(2) the quality of the advertisers (e.g., a document whose advertisements refer/link to documents known to search engine over time to have relatively high traffic and trust, such as amazon.com, may be given relatively more weight than those documents whose advertisements refer to low traffic/untrustworthy documents, such as a pornographic site);
(3) the extent to which the advertisements generate user traffic to the documents to which they relate (e.g., their click-through rate). Search engine may use these time-varying characteristics relating to advertising traffic to score the document.
Information corresponding to individual or aggregate user behavior relating to a document over time may be used to generate (or alter) a score associated with the document. For example, search engine may monitor the number of times that a document is selected from a set of search results and/or the amount of time one or more users spend accessing the document. Search engine may then score the document based, at least in part, on this information.
- If a document is returned for a certain query and over time, or within a given time window, users spend either more or less time on average on the document given the same or similar query, then this may be used as an indication that the document is fresh or stale, respectively. For example, assume that the query "Riverview swimming schedule" returns a document with the title "Riverview Swimming Schedule." Assume further that users used to spend 30 seconds accessing it, but now every user that selects the document only spends a few seconds accessing it. Search engine may use this information to determine that the document is stale (i.e., contains an outdated swimming schedule) and score the document accordingly.
- Information relating to a domain associated with a document may be used to generate (or alter) a score associated with the document. For example, search engine may monitor information relating to how a document is hosted within a computer network (e.g., the Internet, an intranet or other network or database of documents) and use this information to score the document.
- Individuals who attempt to deceive (spam) search engines often use throwaway or "doorway" domains and attempt to obtain as much traffic as possible before being caught. Information regarding the legitimacy of the domains may be used by search engine when scoring the documents associated with these domains.
- Certain signals may be used to distinguish between illegitimate and legitimate domains. For example, domains can be renewed up to a period of 10 years. Valuable (legitimate) domains are often paid for several years in advance, while doorway (illegitimate) domains rarely are used for more than a year. Therefore, the date when a domain expires in the future can be used as a factor in predicting the legitimacy of a domain and, thus, the documents associated therewith.
- The domain name server (DNS) record for a domain may be monitored to predict whether a domain is legitimate. The DNS record contains details of who registered the domain, administrative and technical addresses, and the addresses of name servers (i.e., servers that resolve the domain name into an IP address). By analyzing this data over time for a domain, illegitimate domains may be identified. A list of known-bad contact information, name servers, and/or IP addresses may be identified, stored, and used in predicting the legitimacy of a domain and, thus, the documents associated therewith.
The age, or other information, regarding a name server associated with a domain may be used to predict the legitimacy of the domain. A "good" name server may have a mix of different domains from different registrars and have a history of hosting those domains, while a "bad" name server might host mainly pornography or doorway domains, domains with commercial words (a common indicator of spam), or primarily bulk domains from a single registrar, or might be brand new. The newness of a name server might not automatically be a negative factor in determining the legitimacy of the associated domain, but in combination with other factors, such as ones described herein, it could be.
- Search engine may monitor the time-varying ranking of a document in response to search queries provided to search engine . Search engine may determine that a document that jumps in rankings across many queries might be a topical document or it could signal an attempt to spam search engine. Thus, the quantity or rate that a document moves in rankings over a period of time might be used to influence future scores assigned to that document.
- Search engine may determine that a query is likely commercial if the average (median) score of the top results is relatively high and there is a significant amount of change in the top results from month to month. Search engine may also monitor churn as an indication of a commercial query. For commercial queries, the likelihood of spam is higher, so search engine may treat documents associated therewith accordingly.
- Search engine monitor the average score among a top set of results generated in response to a given query or set of queries and adjust the score of that set of results and/or other results generated in response to the given query or set of queries. Moreover, search engine may monitor the number of results generated for a particular query or set of queries over time. If search engine determines that the number of results increases or that there is a change in the rate of increase (e.g., such an increase may be an indication of a "hot topic" or other phenomenon), search engine may score those results higher in the future.
- search engine may monitor the ranks of documents over time to detect sudden spikes in the ranks of the documents. A spike may indicate either a topical phenomenon (e.g., a hot topic) or an attempt to spam search engine by, for example, trading or purchasing links. Search engine may take measures to prevent spam attempts by, for example, employing hysteresis to allow a rank to grow at a certain rate. In another implementation, the rank for a given document may be allowed a certain maximum threshold of growth over a predefined window of time. As a further measure to differentiate a document related to a topical phenomenon from a spam document, search engine may consider mentions of the document in news articles, discussion groups, etc. on the theory that spam documents will not be mentioned, for example, in the news. Any or a combination of these techniques may be used to curtail spamming attempts.
- It may be possible for search engine to make exceptions for documents that are determined to be authoritative in some respect, such as government documents, web directories (e.g., Yahoo), and documents that have shown a relatively steady and high rank over time. For example, if an unusual spike in the number or rate of increase of links to an authoritative document occurs, then search engine may consider such a document not to be spam and, thus, allow a relatively high or even no threshold for (growth of) its rank (over time).
Search engine may consider significant drops in ranks of documents as an indication that these documents are "out of favor" or outdated. For example, if the rank of a document over time drops significantly, then search engine 125 may consider the document as outdated and score the document accordingly.
User Maintained/Generated Data
- User maintained or generated data may be used to generate (or alter) a score associated with a document. For example, search engine may monitor data maintained or generated by a user, such as "bookmarks," "favorites," or other types of data that may provide some indication of documents favored by, or of interest to, the user. Search engine may obtain this data either directly (e.g., via a browser assistant) or indirectly (e.g., via a browser). Search engine may then analyze over time a number of bookmarks/favorites to which a document is associated to determine the importance of the document.
- Search engine may also analyze upward and downward trends to add or remove the document (or more specifically, a path to the document) from the bookmarks/favorites lists, the rate at which the document is added to or removed from the bookmarks/favorites lists, and/or whether the document is added to, deleted from, or accessed through the bookmarks/favorites lists. If a number of users are adding a particular document to their bookmarks/favorites lists or often accessing the document through such lists over time, this may be considered an indication that the document is relatively important. On the other hand, if a number of users are decreasingly accessing a document indicated in their bookmarks/favorites list or are increasingly deleting/replacing the path to such document from their lists, this may be taken as an indication that the document is outdated, unpopular, etc.
The "temp" or cache files associated with users could be monitored by search engine to identify whether there is an increase or decrease in a document being added over time. Similarly, cookies associated with a particular document might be monitored by search engine to determine whether there is an upward or downward trend in interest in the document.
Linkage of Independent Peers
information regarding linkage of independent peers (e.g., unrelated documents) may be used to generate (or alter) a score associated with a document. A sudden growth in the number of apparently independent peers, incoming and/or outgoing, with a large number of links to individual documents may indicate a potentially synthetic web graph, which is an indicator of an attempt to spam. This indication may be strengthened if the growth corresponds to anchor text that is unusually coherent or discordant. This information can be used to demote the impact of such links, when used with a link-based scoring technique, either as a binary decision item (e.g., demote the score by a fixed amount) or a multiplicative factor.
Information regarding document topics may be used to generate (or alter) a score associated with a document. For example, search engine may perform topic extraction (e.g., through categorization, URL analysis, content analysis, clustering, summarization, a set of unique low frequency words, or some other type of topic extraction). Search engine may then monitor the topic(s) of a document over time and use this information for scoring purposes.
- A significant change over time in the set of topics associated with a document may indicate that the document has changed owners and previous document indicators, such as score, anchor text, etc., are no longer reliable. Similarly, a spike in the number of topics could indicate spam. For example, if a particular document is associated with a set of one or more topics over what may be considered a "stable" period of time and then a (sudden) spike occurs in the number of topics associated with the document, this may be an indication that the document has been taken over as a "doorway" document. Another indication may include the disappearance of the original topics associated with the document. If one or more of these situations are detected, then search engine may reduce the relative score of such documents and/or the links, anchor text, or other data associated the document.
Some of the points appearing in this document can be frightening: Does Google spie on our computers by analyzing our favorites and our cookies? Would Google have a "black list" of hosts and domains? Once again, it should be remembered that this text is a patent: Google filed the idea to analyze the favorites and the cookies. That does not mean that it is legal and that Google applies this idea. By this comment, we do not wish to defend Google in any way but simply to be more specific about the meaning of this text.
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