Lesson 5.8: Link Analysis - Part 3

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來自 University of Illinois at Urbana-Champaign 的課程

Text Retrieval and Search Engines

364 個評分

University of Illinois at Urbana-Champaign

Text Retrieval and Search Engines

364 個評分

課程 2（共 6 門，Specialization Data Mining ）

Recent years have seen a dramatic growth of natural language text data, including web pages, news articles, scientific literature, emails, enterprise documents, and social media such as blog articles, forum posts, product reviews, and tweets. Text data are unique in that they are usually generated directly by humans rather than a computer system or sensors, and are thus especially valuable for discovering knowledge about people’s opinions and preferences, in addition to many other kinds of knowledge that we encode in text. This course will cover search engine technologies, which play an important role in any data mining applications involving text data for two reasons. First, while the raw data may be large for any particular problem, it is often a relatively small subset of the data that are relevant, and a search engine is an essential tool for quickly discovering a small subset of relevant text data in a large text collection. Second, search engines are needed to help analysts interpret any patterns discovered in the data by allowing them to examine the relevant original text data to make sense of any discovered pattern. You will learn the basic concepts, principles, and the major techniques in text retrieval, which is the underlying science of search engines.

從本節課中

Week 5

In this week's lessons, you will learn feedback techniques in information retrieval, including the Rocchio feedback method for the vector space model, and a mixture model for feedback with language models. You will also learn how web search engines work, including web crawling, web indexing, and how links between web pages can be leveraged to score web pages.

Lesson 5.1: Feedback in Text Retrieval6:49

Lesson 5.2: Feedback in Vector Space Model - Rocchio12:05

Lesson 5.3: Feedback in Text Retrieval - Feedback in LM19:11

Lesson 5.4: Web Search: Introduction & Web Crawler11:05

Lesson 5.5: Web Indexing17:19

Lesson 5.6: Link Analysis - Part 19:16

Lesson 5.7: Link Analysis - Part 217:30

Lesson 5.8: Link Analysis - Part 35:59

與講師見面

ChengXiang Zhai
Professor
Department of Computer Science

[SOUND] So

we talked about PageRank as a way to capture the assault.

Now, we also looked at some other examples where a hub might be interesting.

So there is another algorithm called HITS, and

that going to compute the scores for authorities and hubs.

The intuitions are pages that are widely cited are good authorities and

whereas pages that cite many other pages are good hubs.

I think that the most interesting idea of this algorithm HITS,

is it's going to use a reinforcement mechanism

to kind of help improve the scoring for hubs and the authorities.

And so here's the idea,

it was assumed that good authorities are cited by good hubs.

That means if you are cited by many pages with good hub scores then

that inquiry says, you're an authority.

And similarly, good hubs are those that point at good authorities.

So if you pointed to a lot of good authority pages,

then your hubs score would be increased.

So then you will have literally reinforced each other, because you have pointed so

some good hubs.

And so you have pointed to some good authorities to get a good hubs score,

whereas those authority scores would be also

improved because they are pointing to by a good hub.

And this is algorithms is also general it can have many applications in graph and

network analysis.

So just briefly, here's how it works.

We first also construct a matrix, but this time we're going to construct an adjacent

matrix and we're not going to normalize the values.

So if there's a link there's a 1, if there's no link that's 0.

Again, it's the same graph.

And then we're going to define the hubs score of page

as the sum of the authority scores of all the pages that it appoints to.

So whether you are hub,

really depends on whether you are pointing to a lot of good authority pages.

That's what it says in the first equation.

In the second equation, we define the authorities of a page

as a sum of the hub scores of all those pages that appoint to you.

So whether you are good authority would depend on whether those

pages that are pointing to you are good hubs.

So you can see this forms iterative reinforcement mechanism.

Now, these three questions can be also written in the metrics format.

So what we get here is then the hub vector is equal to the product

of the adjacency matrix and the authority vector,

and this is basically the first equation.

And similarly, the second equation can be returned as the authority

vector is equal to the product of a transpose multiplied by the hub vector.

Now, these are just different ways of expressing these equations.

But what's interesting is that if you look at the matrix form,

you can also plug in the authority equation into the first one.

So if you do that, you have actually eliminated the authority vector completely

and you get the equations of only hubs scores.

The hubs score vector is equal to a multiplied

by a transpose multiplied by the hub score again.

Similarly, we can do a transformation to have equation for

just the authorities also.

So although we frame the problem as computing hubs and authorities,

we can actually eliminate one of them to obtain equation just for one of them.

Now, the difference between this and page random is that now the matrix is actually

a multiplication of the adjacency matrix and it's transpose.

So this is different from page rank.

But mathematically, then we will be computing the same problem.

So in HITS, we typically would initialize the values.

Let's say, 1 for all these values, and

then we would iteratively apply these equations, essentially.

And this is equivalent to multiply that by the metrics a and a transpose.

So the arrows of these is exactly the same in the PageRank.

But here because the adjacency matrix is not normalized.

So what we have to do is after each iteration we're going to normalize, and

this would allow us to control the growth of value.

Otherwise they would grow larger and larger.

And if we do that, and that will basically get HITS.

That was the computer, the hubs scores, and authority scores for all the pages.

And these scores can then be used in branching just like the PageRank scores.

So to summarize in this lecture, we have seen that link information's very useful.

In particular, the anchor text is very useful to

increase the text representation of a page.

And we also talk about the PageRank and

page anchor as two major link analysis algorithms.

Both can generate scores for web pages that can be used in the ranking function.

Note that PageRank and the HITS are also very general algorithms.

So they have many applications in analyzing other graphs or networks.

[MUSIC]

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