OCR A-Level Computing - Unit 01 Computer Systems Lesson 1. 3 OCR A-Level Computing - Unit 01 Computer Systems Lesson 1.3.4c – Page Rank Algorithm  Mr C Johnston

What does the image above show? What factors contribute towards result position?

Learning Outcome Understand how the PageRank algorithm influences search result positioning

Search Engine Indexing Google: Inside Search What are the steps involved with getting sites indexed? 1. Search engine ‘spider’ searches the web for web pages. It locates a home page and reads the header page content. It finds and follows links. 2. Data is returned to the search engine database and a list is made of the words found and web pages they were found on. The search engine then indexes the web pages. 3. When you search for something, the search engine looks through this index and returns the websites found in its list. 4. Pages are ranked by an algorithm and the higher the score the higher the web page appears in the results returned. Is a search engine spider actually a ?

PageRank Algorithm Defined Factors which affect the page rank score include: The number of web pages that link to the target page Links to high-ranking web pages score more than those to low- ranking web pages Pages with lots of links add less to the score than those with few links The time that the web page has been in existence also adds to the score; more established sites score more highly than recently published sites All the above can be summarised into this formula PR(A) = (1– d) + d(PR(t1)/C(t1) + ... + PR(tn)/C(tn)) Where t1 to tn are the pages linking to it. C is the number of outbound links on the contributing page and d is a damper of 0.85.

Ranking Example Who is the most popular? All the same one vote each Has two votes but has only voted for one other

PageRank Algorithm Explained PR(A) = (1– d) + d(PR(t1)/C(t1) + ... + PR(tn)/C(tn)) Where t1 to tn are the pages linking to it. C is the number of outbound links on the contributing page and d is a damper of 0.85. Page rank = 0.15 + 0.85 times a share of the page rank for websites that link to it So… to work out the page rank for one page we need to know the page rank of all the pages that link to it, but to work out the page rank of these we need to know the page rank of the original Solution… start with pages with a rank on 1 and iterate the formulae lots of times (some say 40, 50, 100) to get an accurate result

Worked Example A B C Page rank = 0.15 + 0.85 times a share of the page rank for websites that link to it PR(A) = 0.15 + 0.85 * PR(C) = 0.15 + (0.85*1) = 1 PR(B) = 0.15 + 0.85 * (PR(A)/2) = 0.15 + 0.85*(1/2) = 0.15 + 0.425 = 0.575 PR(C) = 0.15 + 0.85 * ((PR(A)/2)+PR(B)) = 0.15 + 0.85 * (1/2 + 0.575) = 0.15 + 0.85 * (1.075) = 0.15 + 0.91375 = 1.06375

Activity A D B C Using the formulae Page rank = 0.15 + 0.85 times a share of the page rank for websites that link to it work out the page ratings for all the pages in this model Design you own page rating problem with answer and ask a peer to complete it A D B C