1. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine Modeling the Internet and the Web: Modeling and Understanding Human Behavior on the Web

2. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 2 Outline Introduction Web Data and Measurement Issues Empirical Client-Side Studies of Browsing Behavior Probabilistic Models of Browsing Behavior Modeling and Understanding Search Engine Querying

3. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 3 Introduction Useful to study human digital behavior, e.g. search engine data can be used for –Exploration e.g. # of queries per session? –Modeling e.g. any time of day dependence? –Prediction e.g. which pages are relevant? Helps –Understand social implications of Web usage –Better design tools for information access –In networking, e-commerce etc

4. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 4 Web data and measurement issues Background: Important to understand how data is collected Web data is collected automatically via software logging tools –Advantage: No manual supervision required –Disadvantage: Data can be skewed (e.g. due to the presence of robot traffic) Important to identify robots (also known as crawlers, spiders)

5. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 5 A time-series plot of Web requests Number of page requests per hour as a function of time from page requests in the www.ics.uci.edu Web server logs during the first week of April 2002.www.ics.uci.edu

6. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 6 Robot / human identification Robot requests identified by classifying page requests using a variety of heuristics –e.g. some robots self-identify themselves in the server logs (robots.txt) –Robots explore the entire website in breadth first fashion –Humans access web-pages in depth first fashion Tan and Kumar (2002) discuss more techniques

7. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 7 Robot / human identification Robot traffic consists of two components –Periodic Spikes (can overload a server) Requests by “bad” robots –Lower-level constant stream of requests Requests by “good” robots Human traffic has –Daily pattern: Monday to Friday –Hourly pattern: peak around midday & low traffic from midnight to early morning

8. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 8 Server-side data Data logging at Web servers Web server sends requested pages to the requester browser It can be configured to archive these requests in a log file recording –URL of the page requested –Time and date of the request –IP address of the requester –Requester browser information (agent)

9. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 9 Data logging at Web servers –Status of the request –Referrer page URL if applicable Server-side log files –provide a wealth of information –require considerable care in interpretation More information in Cooley et al. (1999), Mena (1999) and Shahabi et al. (2001)

10. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 10 Page requests, caching, and proxy servers In theory, requester browser requests a page from a Web server and the request is processed In practice, there are –Other users –Browser caching –Dynamic addressing in local network –Proxy Server caching

11. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 11 Page requests, caching, and proxy servers A graphical summary of how page requests from an individual user can be masked at various stages between the user’s local computer and the Web server.

12. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 12 Page requests, caching, and proxy servers Web server logs are therefore not so ideal in terms of a complete and faithful representation of individual page views There are heuristics to try to infer the true actions of the user: - –Path completion (Cooley et al. 1999) e.g. If known B -> F and not C -> F, then session ABCF can be interpreted as ABCBF Anderson et al. 2001 for more heuristics In general case, hard to know what user viewed

13. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 13 Identifying individual users from Web server logs Useful to associate specific page requests to specific individual users IP address most frequently used Disadvantages –One IP address can belong to several users –Dynamic allocation of IP address Better to use cookies –Information in the cookie can be accessed by the Web server to identify an individual user over time –Actions by the same user during different sessions can be linked together

14. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 14 Identifying individual users from Web server logs Commercial websites use cookies extensively 90% of users have cookies enabled permanently on their browsers However … –There are privacy issues – need implicit user cooperation –Cookies can be deleted / disabled Another option is to enforce user registration –High reliability –Can discourage potential visitors

15. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 15 Client-side data Advantages of collecting data at the client side: –Direct recording of page requests (eliminates ‘masking’ due to caching) –Recording of all browser-related actions by a user (including visits to multiple websites) –More-reliable identification of individual users (e.g. by login ID for multiple users on a single computer) Preferred mode of data collection for studies of navigation behavior on the Web Companies like comScore and Nielsen use client-side software to track home computer users Zhu, Greiner and Häubl (2003) used client-side data

16. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 16 Client-side data Statistics like ‘Time per session’ and ‘Page-view duration’ are more reliable in client-side data Some limitations –Still some statistics like ‘Page-view duration’ cannot be totally reliable e.g. user might go to fetch coffee –Need explicit user cooperation –Typically recorded on home computers – may not reflect a complete picture of Web browsing behavior Web surfing data can be collected at intermediate points like ISPs, proxy servers –Can be used to create user profile and target advertise

17. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 17 Handling massive Web server logs Web server logs can be very large –Small university department website gets a million requests per month –Amazon, Google can get tens of millions of requests each day Exceed main memory capacities, stored on disks Time-costs to data access place significant constraints on types of analysis In practice –Analysis of subset of data –Filtering out events and fields of no direct interest

18. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 18 Empirical client-side studies of browsing behavior Data for client-side studies are collected at the client-side over a period of time –Reliable page revisitation patterns can be gathered –Explicit user permission is required –Typically conducted at universities –Number of individuals is small –Can introduce bias because of the nature of the population being studied –Caution must be exercised when generalizing observations Nevertheless, provide good data for studying human behavior

19. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 19 Early studies from 1995 to 1997 Earliest studies on client-side data are Catledge and Pitkow (1995) and Tauscher and Greenberg (1997) In both studies, data was collected by logging Web browser commands Population consisted of faculty, staff and students Both studies found –clicking on the hypertext anchors as the most common action –using ‘back button’ was the second common action

20. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 20 Early studies from 1995 to 1997 –high probability of page revisitation (~0.58-0.61) Lower bound because the page requests prior to the start of the studies are not accounted for Humans are creatures of habit? Content of the pages changed over time? –strong recency (page that is revisited is usually the page that was visited in the recent past) effect Correlates with the ‘back button’ usage Similar repetitive actions are found in telephone number dialing etc

21. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 21 The Cockburn and McKenzie study from 2002 Previous studies are relatively old Web has changed dramatically in the past few years Cockburn and McKenzie (2002) provides a more up-to-date analysis –Analyzed the daily history.dat files produced by the Netscape browser for 17 users for about 4 months –Population studied consisted of faculty, staff and graduate students Study found revisitation rates higher than past 94 and 95 studies (~0.81) –Time-window is three times that of past studies

22. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 22 The Cockburn and McKenzie study from 2002 Revisitation rate less biased than the previous studies? Human behavior changed from an exploratory mode to a utilitarian mode? –The more pages user visits, the more are the requests for new pages –The most frequently requested page for each user can account for a relatively large fraction of his/her page requests Useful to see the scatter plot of the distinct number of pages requested per user versus the total pages requested Log-log plot also informative

23. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 23 The Cockburn and McKenzie study from 2002 The number of distinct pages visited versus page vocabulary size of each of the 17 users in the Cockburn and McKenzie (2002) study

24. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 24 The Cockburn and McKenzie study from 2002 The number of distinct pages visited versus page vocabulary size of each of the 17 users in the Cockburn and McKenzie (2002) study (log-log plot)

25. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 25 The Cockburn and McKenzie study from 2002 Bar chart of the ratio of the number of page requests for the most frequent page divided by the total number of page requests, for 17 users in the Cockburn McKenzie (2002) study

26. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 26 Video-based analysis of Web usage Byrne et al. (1999) analyzed video-taped recordings of eight different users over a period of 15 min to 1 hour Audio descriptions of the users was combined with the video recordings of their screen for analysis Study found –users spent a considerable amount of time scrolling Web pages –users spent a considerable amount of time waiting for pages to load (~15% of time)

27. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 27 Probabilistic models of browsing behavior Useful to build models that describe the browsing behavior of users Can generate insight into how we use Web Provide mechanism for making predictions Can help in pre-fetching and personalization

28. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 28 Markov models for page prediction General approach is to use a finite-state Markov chain –Each state can be a specific Web page or a category of Web pages –If only interested in the order of visits (and not in time), each new request can be modeled as a transition of states Issues –Self-transition –Time-independence

29. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 29 Markov models for page prediction For simplicity, consider order-dependent, time- independent finite-state Markov chain with M states Let s be a sequence of observed states of length L. e.g. s = ABBCAABBCCBBAA with three states A, B and C. s t is state at position t (1<=t<=L). In general, Under a first-order Markov assumption, we have This provides a simple generative model to produce sequential data

30. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 30 Markov models for page prediction If we denote T ij = P(s t = j|s t-1 = i), we can define a M x M transition matrix Properties –Strong first-order assumption –Simple way to capture sequential dependence If each page is a state and if W pages, O(W 2 ), W can be of the order 10 5 to 10 6 for a CS dept. of a university To alleviate, we can cluster W pages into M clusters, each assigned a state in the Markov model Clustering can be done manually, based on directory structure on the Web server, or automatic clustering using clustering techniques

31. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 31 Markov models for page prediction T ij = P(s t = j|s t-1 = i) now represent the probability that an individual user’s next request will be from category j, given they were in category i We can add E, an end-state to the model E.g. for three categories with end state: - E denotes the end of a sequence, and start of a new sequence

32. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 32 Markov models for page prediction First-order Markov model assumes that the next state is based only on the current state Limitations –Doesn’t consider ‘long-term memory’ We can try to capture more memory with kth- order Markov chain Limitations –Inordinate amount of training data O(M k+1 )

33. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 33 Fitting Markov models to observed page-request data Assume that we collected data in the form of N sessions from server-side logs, where i th session s i, 1<= i <= N, consists of a sequence of L i page requests, categorized into M – 1 states and terminating in E. Therefore, data D = {s 1, …, s N } Let denote the set of parameters of the Markov model, consists of M 2 -1 entries in T Let denote the estimated probability of transitioning from state i to j.

34. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 34 Fitting Markov models to observed page-request data The likelihood function would be This assumes conditional independence of sessions. Under Markov assumptions, likelihood is where n ij is the number of times we see a transition from state i to state j in the observed data D.

35. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 35 Fitting Markov models to observed page-request data For convenience, we use log-likelihood We can maximize the expression by taking partial derivatives wrt each parameter and incorporating the constraint (via Lagrange multipliers) that the sum of transition probabilities out of any state must sum to one The maximum likelihood (ML) solution is

36. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 36 Bayesian parameter estimation for Markov models In practice, M is large (~10 2 -10 3 ), we end up estimating M 2 probabilities D may contain potentially millions of sequences, so some n ij = 0 Better way would be to incorporate prior knowledge – prior probability distribution and then maximize, the posterior distribution on given the data (rather than ) Prior distribution reflects our prior belief about the parameter set The posterior reflects our posterior belief in the parameter set now informed by the data D

37. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 37 Bayesian parameter estimation for Markov models For Markov transition matrices, it is common to put a distribution on each row of T and assume that each of these priors are independent where Consider the set of parameters for the i th row in T, a useful prior distribution on these parameters is the Dirichlet distribution defined as where, and C is a normalizing constant

38. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 38 Bayesian parameter estimation for Markov models The MP posterior parameter estimates are If n ij = 0 for some transition (i, j) then instead of having a parameter estimate of 0 (ML), we will have allowing prior knowledge to be incorporated If n ij > 0, we get a smooth combination of the data-driven information (n ij ) and the prior

39. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 39 Bayesian parameter estimation for Markov models One simple way to set prior parameter is –Consider alpha as the effective sample size –Partition the states into two sets, set 1 containing all states directly linked to state i and the remaining in set 2 –Assign uniform probability e/K to all states in set 2 (all set 2 states are equally likely) –The remaining (1-e) can be either uniformly assigned among set 1 elements or weighted by some measure –Prior probabilities in and out of E can be set based on our prior knowledge of how likely we think a user is to exit the site from a particular state

40. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 40 Predicting page requests with Markov models Many flavors of Markov models proposed for next page and future page prediction Useful in pre-fetching, caching and personalization of Web page For a typical website, the number of pages is large – Clustering is useful in this case First-order Markov models are found to be inferior to other types of Markov models kth-order is an obvious extension –Limitation: O(M k+1 ) parameters (combinatorial explosion)

41. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 41 Predicting page requests with Markov models Deshpande and Karypis (2001) propose schemes to prune kth-order Markov state space –Provide systematic but modest improvements Another way is to use empirical smoothing techniques that combine different models from order 1 to order k (Chen and Goodman 1996) Cadez et al. (2003) and Sen and Hansen (2003) propose mixtures of Markov chains, where we replace the first-order Markov chain:

42. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 42 Predicting page requests with Markov models with a mixture of first-order Markov chains where c is a discrete-value hidden variable taking K values Sum k P(c = k) = 1and P(s t | s t-1, c = k) is the transition matrix for the kth mixture component One interpretation of this is user behavior consists of K different navigation behaviors described by the K Markov chains Cadez et al. use this model to cluster sequences of page requests into K groups, parameters are learned using the EM algorithm

43. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 43 Predicting page requests with Markov models Consider the problem of predicting the next state, given some number of states t Let s [1,t] = {s 1,…, s t } denote the sequence of t states The predictive distribution for a mixture of K Markov models is The last line is obtained if we assume conditioned on component c = k, the next state s t+1 depends only on s t

44. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 44 Predicting page requests with Markov models Weight based on observed history is where Intuitively, these membership weights ‘evolve’ as we see more data from the user In practice, –Sequences are short –Not realistic to assume that observed data is generated by a mixture of K first-order Markov chains Still, mixture model is a useful approximation

45. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 45 Predicting page requests with Markov models K can be chosen by evaluating the out-of- sample predictive performance based on –Accuracy of prediction –Log probability score –Entropy Other variations of Markov models –Sen and Hansen 2003 –Position-dependent Markov models (Anderson et al. 2001, 2002) –Zukerman et al. 1999

46. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 46 Search Engine Querying How users issue queries to search engines –Tracking search query logs timestamp, text string, user ID etc. –Collecting query datasets from different distribution Jansen et al (1998), Silverstein et al (1998) Lau and Horvitz (1999), Spink et al (2002) Xie and O’Hallaron (2002) e.g. Xie and O’Hallaron (2002) Checked how many queries were coming Checked “user’s” IP address Reported 111,000 queries (2.7%) originating from AOL

47. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 47 Analysis of Search Engine Query Logs # of Sample QuerySource SETime Period Lau & Horvitz4690 of 1 MillionExciteSep 1997 Silverstein et al1 BillionAltaVista6 weeks in Aug & Sep 1998 Spink et al(series of studies) 1Million for each time period ExciteSep 1997 Dec 1999 May 2001 Xie & O’Hallaron 110,000Vivisimo35 days Jan & Feb 2001 1.9 MillionExcite8 hrs in a day, Dec 1999

48. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 48 Main Results Average number of terms in a query is ranging from a low of 2.2 to a high of 2.6 The most common number of terms in a query is 2 The majority of users don’t refine their query –The number of users who viewed only a single page increase 29% (1997) to 51% (2001) (Excite) –85% of users viewed only first page of search results (AltaVista) 45% (2001) of queries is about Commerce, Travel, Economy, People (was 20%1997) –The queries about adult or entertainment decreased from 20% (1997) to around 7% (2001)

49. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 49 Main Results All four studies produced a generally consistent set of findings about user behavior in a search engine context –most users view relatively few pages per query –most users don’t use advanced search features - Query Length Distributions (bar) - Poisson Model (dots & lines)

50. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 50 Advanced Search Tips Useful operators for searching (Google) + Include stop word (common words) +where +is Irvine - Exclude operating system -Microsoft ~ Synonyms ~computer “…“ Phrase search “modeling the internet” or Either A Or B vacation London or Paris site: Domain search admission site:www.uci.edu

51. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 51 Power-law Characteristics Frequency f(r) of Queries with Rank r –110000 queries from Vivisimo –1.9 Million queries from Excite There are strong regularities in terms of patterns of behavior in how we search the Web Power-Law in log-log space

52. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 52 Models for Search Strategies It is significant to know the process by which a typical user navigates through search space when looking for information using a search engine The inference of user’s search actions could be used for marketing purposes such as real-time targeted advertising

53. Modeling the Internet and the Web School of Information and Computer Science University of California, Irvine 53 Graphical Representation Lar & Horvitz(1999) –Model of user’s search query actions over time –Simple Bayesian network 1)Current search action 2)Time interval 3)Next search action 4)Informational goals – Track ‘Search Trajectory’ of individual users – Provide more relevant feedback to users