PageRank & the Random Surfer Model

Transcription

1 PageRank & the Random Surfer pg 1 of 4 PageRank & the Random Surfer Model This activity explores how a simulation of randomly surfing the web can be used to rank web search results. I. Pages and Links The figure below represents 4 web pages about places in Europe. Each underlined word represents a link to another page (with the same name). 1. How many links are there on the page for Europe? for Germany? Links like these that lead from one page to other pages are called outbound links. 2. How many pages have links to the page for Europe? for France? Links like these that lead to one page from other pages are called inbound links. 3. On the real internet, which page would you expect to have more incoming links: the homepage of the Portland Trailblazers? Or the homepage for North Salem High Basketball? Explain. Continue

2 PageRank & the Random Surfer pg 2 of 4 The diagram below shows an internet with 6 web pages. Lines connecting pages are links. Each link is an incoming link for one page (the one it points to) and an outgoing link from the other page. 4. How many outgoing links are there from page 3? 5. Which page has the most incoming links? 6. If you are currently on Page 3, which pages are reachable by following one or more links between pages? 7. Why are there two links between pages 1 and 6?

3 PageRank & the Random Surfer pg 3 of 4 This version of the 6-page internet has a range of numbers listed next to each outgoing link. You will use these to randomly select outgoing links while browsing a page. 8. Run the following algorithm until you have made 50 tally marks. The tally marks will be a record of how many times you visit each page. One person should roll the die, one person should keep track of what page you are on and record tally marks, another person should keep track of what step of the algorithm you are on. Start with step 1, after finishing each instruction, if you are not told to go to a different step, move on to the next step. When you are done, count up the tally marks for each page. 1) Roll your die. The number shown is the page you are currently browsing. 2) Record a tally mark on your answer sheet next for that page. 3) Roll your die. If it is a 1, you decided to do a random restart. Go to step 1. 4) Does your page have any outgoing links? If it does not, go to step 1. 5) Roll your die again. Follow the outgoing link that is labeled with that number to a new page. This new page is now the one you are browsing. Go to step 2.

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5 PageRank & the Random Surfer pg 5 of 4 II. Random Surfing & Page Importance A copy of the same internet is shown to the right. Use it and the totaled class data to answer the following questions. 1. There are no inbound links to Page 4. How did you end up there? 2. A page with lots of visits can be described as having authority. Which page had the highest authority? What led to it getting so many visits? 3. What was the probability that when you picked a random outgoing link on page 5, you would end up at page 2? 4. What was the probability that when you picked a random outgoing link on page 2, you would end up at page 5? 5. Page 5 has only one incoming link. Why did it get so many visits? 6. What factors make pages receive more visits by the random surfer algorithm?