Exercise 1.2010.Solutions

581257 Information Retrieval Methods Autumn 2010 Exercise 1, solutions 1. What kind of information retrieval tasks you have completed? What kind of retrieval methods you have used to complete them? Solution: The answer is individual. Methods mentioned may include searching information on a certain topic using some search engine like Google, searching scientific articles with some library search system or from digital libraries, searching for available flights and accommodation with some search engine of a travel agency, browsing in a web site, etc. 2. (Course book's exercise 1.2) Consider these documents: Doc 1 Doc 2 Doc 3 Doc 4

breakthrough drug for schizophrenia new schizophrenia drug new approach for treatment of schizophrenia new hopes for schizophrenia patients

a) Draw the term-document incidence matrix for this document collection. Solution: Term-document matrix: D1

D2

D3

D4

approach

0

0

1

0

breakthrough

1

0

0

0

drug

1

1

0

0

for

1

0

1

1

hopes

0

0

0

1

new

0

1

1

1

of

0

0

1

0

patients

0

0

0

1

schizophrenia

1

1

1

1

treatment

0

0

1

0

b) Draw the inverted index representation for this collection, as in Figure 1.3 (page 6). Solution: Inverted index: approach → 3 breakthrough → 1 drug → 1 → 2 for → 1 → 3 → 4 hopes → 4 new → 2 → 3 → 4 of → 3 patients → 4 schizophrenia → 1 → 2 → 3 → 4 treatment → 3

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3. (Course book's exercise 1.3) For the document collection shown in Exercise 2 (Course book's exercise 1.2), what are the returned results for these queries: a) schizophrenia AND drug Solution: doc1, doc2 b) for AND NOT (drug OR approach) Solution: doc4 4. (Course book's exercise 1.4) For the queries below, can we still run through the intersection in time O(x+y), where x and y are the lengths of the postings lists for Brutus and Caesar? If not, what can we achieve? a) Brutus AND NOT Caesar Solution: Time is O(x+y). Instead of collecting documents that occur in both postings lists, collect those that occur in the first one and not in the second. b) Brutus OR NOT Caesar Solution: Time is O(N) (where N is the total number of documents in the collection) assuming we need to return a complete list of all documents satisfying the query. This is because the length of the results list is only bounded by N, not by the length of the postings lists. 5. (Course book's exercise 1.7) Recommend a query processing order for (tangerine OR trees) AND (marmalade OR skies) AND (kaleidoscope OR eyes) given the following postings list sizes: Term

Postings size

eyes

213312

kaleidoscope

87009

marmalade

107913

skies

271658

tangerine

46653

trees

316812

Solution: Using the conservative estimate of the length of the union of postings lists, the recommended order is: (kaleidoscope OR eyes) (300,321) AND (tangerine OR trees) (363,465) AND (marmalade OR skies) (379,571) However, depending on the actual distribution of postings, (tangerine OR trees) may well be longer than (marmalade OR skies), because the two components of the former are more asymmetric. For example, the union of 11 and 9990 is expected to be longer than the union of 5000 and 5000 even though the conservative estimate predicts otherwise.

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6. Try the search feature at http://www.rhymezone.com/shakespeare/. Write down five search features you think it could do better. Solution: Such features may include excluding search terms (NOT), other Boolean operators, ranking based on the relevance, searching for a combination of single word and phrases, wild-card queries, proximity queries, ….. 7. (Course book's exercise 1.13) Try using the Boolean search features on a couple of major web search engines. For instance, choose a word, such as burglar, and submit the queries (i) burglar, (ii) burglar AND burglar, and (iii) burglar OR burglar. Look at the estimated number of results and top hits. Do they make sense in terms of Boolean logic? Often they haven't for major search engines. Can you make sense of what is going on? What about if you try different words? For example, query for (i) knight, (ii) conquer, and then (iii) knight OR conquer. What bound should the number of results from the first two queries place on the third query? Is this bound observed? Example solution: Observations on queries with some popular search engines: Google burglar

Altavista

Bing

8,230,000 2,660,000 - 2,990,000 36,800,000 2,410,000

burglar AND burglar 5,350,000 burglar OR burglar

Yahoo

4,980,000

689,000 - 880,000

36,800,000 4,990,000

3,960,000

36,900,000 2,460,000

The number of hits may slightly change if the same query is run several times. The numeric results depend on which search engine version you are using (e.g., google.com or google.fi). Inferences: When the operator AND is used in a query, Google prompts not to use that because it includes all terms of a query by default. Yahoo and Bing seem to treat the term AND as a normal term and not as a Boolean operator as shown by the top results and the total number of hits. Altavista seems to follow the Boolean logic most tightly. In case of Google, the top documents returned with AND had the term burglar more than once which tells that query processing does not include redundant terms removal. Surprisingly, Bing returns twice the number of documents with the term AND – it would be interesting to hear, what's the reason for that. The term OR is treated as a Boolean operator by Altavista, Google and Yahoo, but the number of hits is still a bit confusing in the case of Google. Observations: Google

Yahoo

Altavista

Bing

knight

83,700,00

6,930,000

495,000,000 4,800,000

conquer

19,800,000

8,300,000

144,000,000 4,620,000

knight and conquer

1,830,000

2,950,000

27,800,000

2,940,000

knight AND conquer 1,830,000

3,170,000

27,800,000

3,030,000

knight or conquer

1,830,000

2,700,000

30,100,000

2,930,000

knight OR conquer

212,000,000 12,600,000 617,000,000 8,860,000

Inferences: The number of hits for knight AND conquer should be less than for knight or conquer separately which is observed in all 4 searches. The number of hits for knight OR conquer should be more than for knight or conquer separately, which is observed in all 4 searches.

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