References -- SCIT1020: How Google used Statistics to Change the World
PageRank–HowGoogleusedStatisticstoChangetheWorld:
Larry Page and Sergey Brin
The story behind Google’s Search Engine
PageRank is what Google uses to determine the importance of a web page. It's one of many factors used
to determine which pages appear in search results.
The History of PageRank
PageRank was developed by Google founders Larry Page and Sergey Brin at Stanford. At the time that Page
and Brin met, search engines typically linked to pages that had the highest keyword density, which meant
people could game the system by repeating the same phrase over and over to attract higher search page
results.
http://google.about.com/od/searchengineoptimization/a/pagerankexplain.htm
http://www.google.com/about/company/history/
http://www.notablebiographies.com/news/Ow-Sh/Page-Larry-and-Brin-Sergey.html
Google founders Sergey Brin and Larry Page first met in 1995 when Page visited the computer science
department of Stanford University during a weekend. Brin was already enrolled in Stanford's PhD program
and a second year graduate student at the time when Page arrived. Brin served as a guide for potential
doctoral students and was assigned to a team that showed them around campus. Page was in Brin’s group.
Industry lore claims they discussed many topics during their first meeting and disagreed on nearly every
issue.
Soon after beginning graduate study at Stanford, Page began working on a Web project, initially called
BackRub, that exploited the link structure of the Web. Brin found Page’s work on BackRub interesting, so
the two started working together on a project that would permanently change Web search.
As Brin explained to Robert McGarvey of Technology Review later, "I was working on data mining, the
idea of taking large amounts of data, analyzing it for patterns and trying to extract relationships that are
useful." Their 1996 paper, "Anatomy of a Large-Scale Hypertextual Web Search Engine," became the basis
for the Google search engine.
Brin and Page realized that they were creating a search engine that adapted to the ever increasing size of the
1
References -- SCIT1020: How Google used Statistics to Change the World
Web, so they replaced the name BackRub with Google (a common misspelling of googol, the number 10100).
Unable to convince existing search engine companies to adopt the technology they had developed but
certain their technology was superior to any being used, Brin and Page decided to start their own company.
With the financial assistance of a small group of initial investors, Brin and Page founded the Web search
engine company Google, Inc. in September 1998.
The web is a gigantic “Graph”
• The web itself can be viewed as a very large
graph
• Nodes are individual sites or pages and edges are
the links between pages (directed graph)
• This is the basis of Google’s page rank algorithm
• The “importance” of a site is determined by the
number of sites that link to it and weighted by the
importance of those sites
• Importance “propagates” around the graph until
it stabilizes, eventually we end up with probability
that a random web surfer will be at a given page
• We can also view other types information as a
graph, for example citations of papers
Almost immediately, the general public noticed what Brin, Page, and others in the academic Web search
community already knew — the Google search engine produced much higher quality results than those
produced by other Web search engines. Other search engines relied entirely on webpage content to
determine ranking of results, and Brin and Page realized that webpage developers could easily manipulate
the ordering of search results by placing concealed information on webpages. Brin and Page developed a
ranking algorithm, named PageRank after Larry Page, that uses the link structure of the Web to determine
the importance of webpages. During the processing of a query, Google’s search algorithm combined
precomputed PageRank scores with text matching scores to obtain an overall ranking score for each
webpage.
Mathematically, a graph is an ordered pair G = (V, E) comprising a set V of vertices or nodes
together with a set E of edges or lines. Graph theory is the branch of mathematics that
devotes to the study of graphs. Besides the PageRank in the Google search engine, it is also
useful in quantitative or qualitative analysis of social networks.
Example: A simple graph with vertex set V = {1, 2, 3, 4, 5, 6} and edge set E = {{1,2}, {1,5}, {2,3}, {2,5}, {3,4}, {4,5}, {4,6}}.
2
References -- SCIT1020: How Google used Statistics to Change the World
Social Network is a social structure, normally represented as a “graph” with:
• Individuals (or organizations) as nodes
• Relationships as edges
We can learn a lot about people from studying their social network - Social Network Analysis.
Applications of Social Network Analysis
• Marketing and fashion - to uncover new trends and major influencers;
• Networking - finding an optimal way of constructing a computer network, locate points of failure and
bottlenecks;
• Intelligence - identifying criminal/spy networks and determining leaders and active cells;
• Epidemiology - to understand how patterns of human contact affect the spread of diseases, such as HIV
... and more.
Power in a network: a historical case
• In 15th century Florence, Italy, the Medici family emerged as the most powerful and ended up
dominating trade in the area
• The Medici family was less powerful than many of the other important families in the beginning, so how
did they achieve so much?
• It has been proposed* that it was their position in the Florentine social network that propelled their
success
* “Robust Action and the Rise of the Medici” Padgett and Ansell (1993)
3
References -- SCIT1020: How Google used Statistics to Change the World
4
References -- SCIT1020: How Google used Statistics to Change the World
5