Prasad L08Evaluation 1

Evaluation of IR Systems

Adapted from Lectures by

Prabhakar Raghavan (Yahoo and Stanford) and Christopher Manning (Stanford)

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This lecture

How do we summarize results? Making our good results usable to a user

How do we know if our results are any good? Necessary to determine effectiveness of

Ranking function (dot-product, cosine, …) Term selection (stopword removal, stemming…) Term weighting (TF, TF-IDF,…)

How far down the ranked list will a user need to look to find some/all relevant documents?

Evaluating a search engine Benchmarks Precision and recall

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Results summaries

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Summaries

Having ranked the documents matching a query, we wish to present a results list (answer set)

Most commonly, the document title plus a short summary

The title is typically automatically extracted from document metadata.

What about the summaries?

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Summaries

Two basic kinds: Static (cf. extractive) Dynamic (cf. abstractive)

A static summary of a document is always the same, regardless of the query that hit the doc.

Dynamic summaries are query-dependent and attempt to explain why the document was retrieved for the query at hand.

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Static summaries In typical systems, the static summary is a subset of the

document. Simplest heuristic: the first 50 (or so – this can be varied)

words of the document Summary cached at indexing time

More sophisticated: extract from each document a set of “key” sentences

Simple NLP heuristics to score each sentence. Summary is made up of top-scoring sentences.

Most sophisticated: NLP used to synthesize a summary Seldom used in IR; (cf. text summarization work)

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Dynamic summaries

Present one or more “windows” within the document that contain several of the query terms “KWIC” snippets: Keyword in Context presentation

Generated in conjunction with scoring If query found as a phrase, the/some occurrences of

the phrase in the doc If not, windows within the doc that contain multiple

query terms The summary itself gives the entire content of the

window – all terms, not only the query terms.

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Generating dynamic summaries

If we have only a positional index, we cannot (easily) reconstruct context surrounding hits

If we cache the documents at index time, can run the window through it, cueing to hits found in the positional index E.g., positional index says “the query is a phrase

in position 4378” so we go to this position in the cached document and stream out the content

Most often, cache a fixed-size prefix of the doc

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Dynamic summaries

Producing good dynamic summaries is a tricky optimization problem

The real estate for the summary is normally small and fixed Want short item, so show as many KWIC matches as

possible, and perhaps other things like title Want snippets to be long enough to be useful Want linguistically well-formed snippets: users prefer snippets

that contain complete phrases Want snippets maximally informative about doc

But users really like snippets, even if they complicate IR system design

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Evaluating search engines

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Measures for a search engine

How fast does it index Number of documents/hour (Average document size)

How fast does it search Latency as a function of index size

Expressiveness of query language Ability to express complex information needs Speed on complex queries

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Measures for a search engine

All of the preceding criteria are measurable: we can quantify speed/size; we can make expressiveness precise

The key measure: user happiness What is this? Speed of response/size of index are factors But blindingly fast, useless answers won’t make a

user happy Need a way of quantifying user happiness

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Data Retrieval vs Information Retrieval

DR Performance Evaluation (after establishing correctness) Response time Index space

IR Performance Evaluation + How relevant is the answer set? (required to

establish functional correctness, e.g., through benchmarks)

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Measuring user happiness Issue: who is the user we are trying to make

happy? Web engine: user finds what they want and return

to the engine Can measure rate of return users

eCommerce site: user finds what they want and make a purchase Is it the end-user, or the eCommerce site, whose

happiness we measure? Measure time to purchase, or fraction of searchers

who become buyers?

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Measuring user happiness

Enterprise (company/govt/academic): Care about “user productivity” How much time do my users save when looking for

information? Many other criteria having to do with breadth of

access, secure access, etc.

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Happiness: elusive to measure

Commonest proxy: relevance of search results But how do you measure relevance? We will detail a methodology here, then examine

its issues Relevance measurement requires 3 elements:

1. A benchmark document collection

2. A benchmark suite of queries

3. A binary assessment of either Relevant or Irrelevant for each query-doc pair Some work on more-than-binary, but not the standard

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Evaluating an IR system

Relevance is assessed relative to the information need, not the query

E.g., Information need: I'm looking for information on whether drinking red wine is more effective at reducing your risk of heart attacks than white wine.

Query: wine red white heart attack effective You evaluate whether the doc addresses the

information need, not whether it has those words

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Difficulties with gauging Relevancy

Relevancy, from a human standpoint, is: Subjective: Depends upon a specific

user’s judgment. Situational: Relates to user’s current

needs. Cognitive: Depends on human

perception and behavior. Dynamic: Changes over time.

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Standard relevance benchmarks

TREC - National Institute of Standards and Testing (NIST) has run a large IR test bed for many years

Reuters and other benchmark doc collections used

“Retrieval tasks” specified sometimes as queries

Human experts mark, for each query and for each doc, Relevant or Irrelevant or at least for subset of docs that some system

returned for that query

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Unranked retrieval evaluation:Precision and Recall

Precision: fraction of retrieved docs that are relevant = P(relevant|retrieved)

Recall: fraction of relevant docs that are retrieved = P(retrieved|relevant)

Precision P = tp/(tp + fp) Recall R = tp/(tp + fn)

Relevant Not Relevant

Retrieved tp fp

Not Retrieved fn tn

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Precision and Recall in Practice

Precision The ability to retrieve top-ranked documents that

are mostly relevant. The fraction of the retrieved documents that is relevant.

Recall The ability of the search to find all of the relevant

items in the corpus. The fraction of the relevant documents that is retrieved.

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Precision and Recall

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R=3/6=0.5; P=3/4=0.75

Computing Recall/Precision Points: An Example

n doc # relevant

1 588 x2 589 x3 5764 590 x5 9866 592 x7 9848 9889 57810 98511 10312 59113 772 x14 990

Let total # of relevant docs = 6Check each new recall point:

R=1/6=0.167; P=1/1=1

R=2/6=0.333; P=2/2=1

R=5/6=0.833; p=5/13=0.38

R=4/6=0.667; P=4/6=0.667

Missing one relevant document.

Never reach 100% recall

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Accuracy

Given a query, an engine classifies each doc as “Relevant” or “Irrelevant”.

Accuracy of an engine: the fraction of these classifications that is correct.

Why is this not a very useful evaluation measure in IR?

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Why not just use accuracy?

How to build a 99.9999% accurate search engine on a low budget….

People doing information retrieval want to find something and have a certain tolerance for junk.

Search for:

0 matching results found.

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Precision/Recall

You can get high recall (but low precision) by retrieving all docs for all queries!

Recall is a non-decreasing function of the number of docs retrieved

In a good system, precision decreases as either number of docs retrieved or recall increases A fact with strong empirical confirmation

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Trade-offs

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1

Recall

Pre

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The idealReturns relevant documents butmisses many useful ones too

Returns most relevantdocuments but includeslot of junk

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Difficulties in using precision/recall

Should average over large corpus/query ensembles

Need human relevance assessments People aren’t reliable assessors

Assessments have to be binary Nuanced assessments?

Heavily skewed by corpus/authorship Results may not translate from one domain to

another

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A combined measure: F

Combined measure that assesses this tradeoff is F measure (weighted harmonic mean):

People usually use balanced F1 measure i.e., with = 1 or = ½

Harmonic mean is a conservative average See CJ van Rijsbergen, Information Retrieval

RP

PR

RP

F

2

2 )1(1

)1(1

1

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F1 and other averages

Combined Measures

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0 20 40 60 80 100

Precision (Recall fixed at 70%)

Minimum

Maximum

Arithmetic

Geometric

Harmonic

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Evaluating ranked results

Evaluation of ranked results: The system can return any number of results By taking various numbers of the top returned

documents (levels of recall), the evaluator can produce a precision-recall curve

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A precision-recall curve

0.0

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0.0 0.2 0.4 0.6 0.8 1.0

Recall

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Evaluation

Graphs are good, but people want summary measures! Precision at fixed retrieval level

Perhaps most appropriate for web search: all people want are good matches on the first one or two results pages

But has an arbitrary parameter of k 11-point interpolated average precision

The standard measure in the TREC competitions: you take the precision at 11 levels of recall varying from 0 to 1 by tenths of the documents, using interpolation (the value for 0 is always interpolated!), and average them

Evaluates performance at all recall levels

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Typical (good) 11 point precisions

SabIR/Cornell 8A1 11pt precision from TREC 8 (1999)

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Recall

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11 point precisions

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0 20 40 60 80 100 120

Recall

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Creating Test Collectionsfor IR Evaluation

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Test Corpora

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From corpora to test collections

Still need Test queries Relevance assessments

Test queries Must be germane to docs available Best designed by domain experts Random query terms generally not a good idea

Relevance assessments Human judges, time-consuming Are human panels perfect?

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Unit of Evaluation

We can compute precision, recall, F, and ROC curve for different units.

Possible units Documents (most common) Facts (used in some TREC evaluations) Entities (e.g., car companies)

May produce different results. Why?

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Can we avoid human judgment?

Not really Makes experimental work hard

Especially on a large scale In some very specific settings, can use proxies Example below, approximate vector space

retrieval But once we have test collections, we can reuse

them (so long as we don’t overtrain too badly)

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Approximate vector retrieval

Given n document vectors and a query, find the k doc vectors closest to the query. Exact retrieval – we know of no better way than to

compute cosines from the query to every doc Approximate retrieval schemes – such as cluster

pruning Given such an approximate retrieval scheme,

how do we measure its goodness?

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Approximate vector retrieval

Let G(q) be the “ground truth” of the actual k closest docs on query q

Let A(q) be the k docs returned by approximate algorithm A on query q

For performance we would measure A(q) G(q) Is this the right measure?

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Alternative proposal

Focus instead on how A(q) compares to G(q). Goodness can be measured here in cosine

proximity to q: we sum up qd over d A(q). Compare this to the sum of qd over d G(q).

Yields a measure of the relative “goodness” of A vis-à-vis G.

Thus A may be 90% “as good as” the ground-truth G, without finding 90% of the docs in G.

For scored retrieval, this may be acceptable: Most web engines don’t always return the same

answers for a given query.

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Other Evaluation Measures andTREC Benchmarks

Adapted from Slides Attributed to

Prof. Dik Lee (Univ. of Science and Tech, Hong Kong)

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R- Precision

Precision at the R-th position in the ranking of results for a query that has R relevant documents.

n doc # relevant

1 588 x2 589 x3 5764 590 x5 9866 592 x7 9848 9889 57810 98511 10312 59113 772 x14 990

R = # of relevant docs = 6

R-Precision = 4/6 = 0.67

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E Measure (parameterized F Measure)

A variant of F measure that allows weighting emphasis on precision over recall:

Value of controls trade-off: = 1: Equally weight precision and recall (E=F). > 1: Weight precision more. < 1: Weight recall more.

PRRP

PRE

1

2

2

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)1()1(

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Fallout Rate

Problems with both precision and recall: Number of irrelevant documents in the

collection is not taken into account. Recall is undefined when there is no relevant

document in the collection. Precision is undefined when no document is

retrieved.

collection the in items tnonrelevan of no. totalretrieved items tnonrelevan of no.

Fallout

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Subjective Relevance Measure

Novelty Ratio: The proportion of items retrieved and judged relevant by the user and of which they were previously unaware.

Ability to find new information on a topic. Coverage Ratio: The proportion of relevant items retrieved

out of the total relevant documents known to a user prior to the search.

Relevant when the user wants to locate documents which they have seen before (e.g., the budget report for Year 2000).

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Other Factors to Consider

User effort: Work required from the user in formulating queries, conducting the search, and screening the output.

Response time: Time interval between receipt of a user query and the presentation of system responses.

Form of presentation: Influence of search output format on the user’s ability to utilize the retrieved materials.

Collection coverage: Extent to which any/all relevant items are included in the document corpus.

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Previous experiments were based on the SMART collection which is fairly small. (ftp://ftp.cs.cornell.edu/pub/smart)

Collection Number Of Number Of Raw Size Name Documents Queries (Mbytes) CACM 3,204 64 1.5 CISI 1,460 112 1.3 CRAN 1,400 225 1.6 MED 1,033 30 1.1 TIME 425 83 1.5

Different researchers used different test collections and evaluation techniques.

Early Test Collections

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The TREC Benchmark

• TREC: Text REtrieval Conference (http://trec.nist.gov/) Originated from the TIPSTER program sponsored by Defense Advanced Research Projects Agency (DARPA).• Became an annual conference in 1992, co-sponsored by the National Institute of Standards and Technology (NIST) and DARPA.• Participants are given parts of a standard set of documents and TOPICS (from which queries have to be derived) in different stages for training and testing.• Participants submit the P/R values for the final document and query corpus and present their results at the conference.

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The TREC Objectives • Provide a common ground for comparing different IR techniques.

– Same set of documents and queries, and same evaluation method.• Sharing of resources and experiences in developing the benchmark.

– With major sponsorship from government to develop large benchmark collections.

• Encourage participation from industry and academia.• Development of new evaluation techniques, particularly for new applications.

– Retrieval, routing/filtering, non-English collection, web-based collection, question answering.

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TREC Advantages

Large scale (compared to a few MB in the SMART Collection).

Relevance judgments provided. Under continuous development with support from the

U.S. Government. Wide participation:

TREC 1: 28 papers 360 pages. TREC 4: 37 papers 560 pages. TREC 7: 61 papers 600 pages. TREC 8: 74 papers.

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TREC Tasks Ad hoc: New questions are being asked on a

static set of data. Routing: Same questions are being asked,

but new information is being searched. (news clipping, library profiling).

New tasks added after TREC 5 - Interactive, multilingual, natural language, multiple database merging, filtering, very large corpus (20 GB, 7.5 million documents), question answering.

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Characteristics of the TREC Collection

Both long and short documents (from a few hundred to over one thousand unique terms in a document).

Test documents consist of: WSJ Wall Street Journal articles (1986-1992) 550 M AP Associate Press Newswire (1989) 514 M ZIFF Computer Select Disks (Ziff-Davis Publishing) 493 M FR Federal Register 469 M DOE Abstracts from Department of Energy reports 190 M

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More Details on Document Collections

Volume 1 (Mar 1994) - Wall Street Journal (1987, 1988, 1989), Federal Register (1989), Associated Press (1989), Department of Energy abstracts, and Information from the Computer Select disks (1989, 1990)

Volume 2 (Mar 1994) - Wall Street Journal (1990, 1991, 1992), the Federal Register (1988), Associated Press (1988) and Information from the Computer Select disks (1989, 1990)

Volume 3 (Mar 1994) - San Jose Mercury News (1991), the Associated Press (1990), U.S. Patents (1983-1991), and Information from the Computer Select disks (1991, 1992)

Volume 4 (May 1996) - Financial Times Limited (1991, 1992, 1993, 1994), the Congressional Record of the 103rd Congress (1993), and the Federal Register (1994).

Volume 5 (Apr 1997) - Foreign Broadcast Information Service (1996) and the Los Angeles Times (1989, 1990).

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TREC Disk 4,5TREC Disk 4 Congressional Record of the 103rd Congress

approx. 30,000 documents approx. 235 MBFederal Register (1994) approx. 55,000 documents approx. 395 MBFinancial Times (1992-1994) approx. 210,000 documents approx. 565 MB

TREC Disk 5 Data provided from the Foreign Broadcast Information Service approx. 130,000 documents approx. 470 MBLos Angeles Times (randomly selected articles from 1989 & 1990) approx. 130,000 document approx. 475 MB

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Sample Document (with SGML)<DOC> <DOCNO> WSJ870324-0001 </DOCNO> <HL> John Blair Is Near Accord To Sell Unit, Sources Say </HL> <DD> 03/24/87</DD> <SO> WALL STREET JOURNAL (J) </SO><IN> REL TENDER OFFERS, MERGERS, ACQUISITIONS (TNM) MARKETING,

ADVERTISING (MKT) TELECOMMUNICATIONS, BROADCASTING, TELEPHONE, TELEGRAPH (TEL) </IN>

<DATELINE> NEW YORK </DATELINE> <TEXT> John Blair &amp; Co. is close to an agreement to sell its TV station advertising

representation operation and program production unit to an investor group led by James H. Rosenfield, a former CBS Inc. executive, industry sources said. Industry sources put the value of the proposed acquisition at more than $100 million. ...

</TEXT> </DOC>

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Sample Query (with SGML)<top> <head> Tipster Topic Description <num> Number: 066 <dom> Domain: Science and Technology <title> Topic: Natural Language Processing <desc> Description: Document will identify a type of natural language processing

technology which is being developed or marketed in the U.S. <narr> Narrative: A relevant document will identify a company or institution

developing or marketing a natural language processing technology, identify the technology, and identify one of more features of the company's product.

<con> Concept(s): 1. natural language processing ;2. translation, language, dictionary

<fac> Factor(s): <nat> Nationality: U.S.</nat></fac> <def> Definitions(s): </top>

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TREC Properties

Both documents and queries contain many different kinds of information (fields).

Generation of the formal queries (Boolean, Vector Space, etc.) is the responsibility of the system. A system may be very good at querying and

ranking, but if it generates poor queries from the topic, its final P/R would be poor.

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Two more TREC Document Examples

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Another Example of TREC Topic/Query

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Evaluation

Summary table statistics: Number of topics, number of documents retrieved, number of relevant documents.

Recall-precision average: Average precision at 11 recall levels (0 to 1 at 0.1 increments).

Document level average: Average precision when 5, 10, .., 100, … 1000 documents are retrieved.

Average precision histogram: Difference of the R-precision for each topic and the average R-precision of all systems for that topic.

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Cystic Fibrosis (CF) Collection

1,239 abstracts of medical journal articles on CF. 100 information requests (queries) in the form of

complete English questions. Relevant documents determined and rated by 4

separate medical experts on 0-2 scale: 0: Not relevant. 1: Marginally relevant. 2: Highly relevant.

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CF Document Fields

MEDLINE access number Author Title Source Major subjects Minor subjects Abstract (or extract) References to other documents Citations to this document

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Sample CF DocumentAN 74154352AU Burnell-R-H. Robertson-E-F.TI Cystic fibrosis in a patient with Kartagener syndrome.SO Am-J-Dis-Child. 1974 May. 127(5). P 746-7.MJ CYSTIC-FIBROSIS: co. KARTAGENER-TRIAD: co.MN CASE-REPORT. CHLORIDES: an. HUMAN. INFANT. LUNG: ra. MALE. SITUS-INVERSUS: co, ra. SODIUM: an. SWEAT: an.AB A patient exhibited the features of both Kartagener syndrome and cystic fibrosis. At most, to the authors' knowledge, this represents the third such report of the combination. Cystic fibrosis should be excluded before a diagnosis of Kartagener syndrome is made.RF 001 KARTAGENER M BEITR KLIN TUBERK 83 489 933 002 SCHWARZ V ARCH DIS CHILD 43 695 968 003 MACE JW CLIN PEDIATR 10 285 971 …CT 1 BOCHKOVA DN GENETIKA (SOVIET GENETICS) 11 154 975 2 WOOD RE AM REV RESPIR DIS 113 833 976 3 MOSSBERG B MT SINAI J MED 44 837 977 …

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Sample CF QueriesQN 00002QU Can one distinguish between the effects of mucus hypersecretion and infection on the submucosal glands of the respiratory tract in CF?NR 00007RD 169 1000 434 1001 454 0100 498 1000 499 1000 592 0002 875 1011

QN 00004QU What is the lipid composition of CF respiratory secretions?NR 00009RD 503 0001 538 0100 539 0100 540 0100 553 0001 604 2222 669 1010 711 2122 876 2222

NR: Number of Relevant documentsRD: Relevant Documents

Ratings code: Four 0-2 ratings, one from each expert