lect17

# lect17 - More on Rankings Query-independent LAR Have an...

This preview shows pages 1–11. Sign up to view the full content.

More on Rankings

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Query-independent LAR Have an a-priori ordering of the web pages Q : Set of pages that contain the keywords in the query q Present the pages in Q ordered according to order π What are the advantages of such an approach?
InDegree algorithm Rank pages according to in-degree w i = |B(i)| 1. Red Page 1. Yellow Page 1. Blue Page 1. Purple Page 1. Green Page w=1 w=1 w=2 w=3 w=2

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
PageRank algorithm [BP98] Good authorities should be pointed by good authorities Random walk on the web graph pick a page at random with probability 1- α jump to a random page with probability α follow a random outgoing link Rank according to the stationary distribution 1. Red Page 1. Purple Page   1. Yellow Page 1. Blue Page 1. Green Page ( 29 n q F q PR p PR p q 1 1 ) ( ) ( ) ( α - + =
Markov chains A Markov chain describes a discrete time stochastic process over a set of states according to a transition probability matrix

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Random walks Random walks on graphs correspond to Markov Chains The set of states S is the set of nodes of the graph G The transition probability matrix is the probability that we follow an edge from one node to another
An example v 1 v 2 v 3 v 4 v 5 = 2 1 0 0 0 2 1 0 0 3 1 3 1 3 1 0 0 0 1 0 1 0 0 0 0 0 0 2 1 2 1 0 P = 1 0 0 0 1 0 0 0 1 0 1 0 0 0 0 0 0 1 1 0 A

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
State probability vector
An example = 0 2 1 0 0 2 1 0 0 3 1 3 1 3 1 0 0 0 1 0 1 0 0 0 0 0 0 2 1 2 1 0 P v 1 v 2 v 3 v 4 v 5 q = 1/3 q + 1/2 q t 5 q = 1/2 q t 5

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Stationary distribution
This is the end of the preview. Sign up to access the rest of the document.

## This document was uploaded on 10/05/2010.

### Page1 / 30

lect17 - More on Rankings Query-independent LAR Have an...

This preview shows document pages 1 - 11. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online