Web Engineering & Development Lecture 5: Search Engines
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Questions and Answers

What is the purpose of rotating a wildcard query?

  • To ensure the * symbol is at the end of the string (correct)
  • To improve search speed
  • To make the query more complex
  • To eliminate irrelevant terms

    • The term 'fishmonger' would not survive the filtering process when checking for the substring 'mo'.

    False

    What is the lookup key for the wildcard query 'uni*rse'?

    rse$uni*

    The process of creating an inverted index from bigrams involves enumerating all ______ occurring in any term.

    <p>k-grams</p> Signup and view all the answers

    Match the query to its corresponding lookup key:

    <p>uni<em>rse = rse$uni</em> fi<em>mo</em>er = er$fi* uni<em>e</em>se = se$uni* mon* = $m AND mo AND on</p> Signup and view all the answers

    What does the * symbol indicate in wildcard queries?

    <p>Any (possibly empty) string of characters</p> Signup and view all the answers

    Permuterm index is a technique used to handle general wildcard queries.

    <p>True</p> Signup and view all the answers

    Give an example of a wildcard query that seeks terms containing all five vowels in sequence.

    <p><em>a</em>e<em>i</em>o<em>u</em></p> Signup and view all the answers

    A wildcard query such as automat* seeks documents containing variants of the query term such as __________, __________, and __________.

    <p>automatic, automation, automated</p> Signup and view all the answers

    Match the techniques used to handle wildcard queries with their explanations:

    <p>Permuterm index = Rotate queries with * at the end K-gram index = Break terms into segments of k characters TF-IDF weights = Rank documents based on term frequency Precision &amp; Recall = Evaluate the effectiveness of search results</p> Signup and view all the answers

    What is the purpose of the K-gram index?

    <p>To break terms into segments of k characters</p> Signup and view all the answers

    Edit distance is a method used for ranking documents.

    <p>False</p> Signup and view all the answers

    What common strategy do the Permuterm index and K-gram index share?

    <p>Express the given wildcard query as a Boolean query on a specially constructed index.</p> Signup and view all the answers

    The edit distance algorithm is also known as Hamming distance.

    <p>False</p> Signup and view all the answers

    What is the edit distance between the words 'cat' and 'dog'?

    <p>3</p> Signup and view all the answers

    Given a lexicon and a character sequence Q, the spelling correction method can return the words closest to ___

    <p>Q</p> Signup and view all the answers

    Match the following terms with their correct definitions:

    <p>Edit Distance = Minimum operations to convert one string to another Levenshtein Distance = Another name for edit distance Dynamic Programming = A method used to calculate the edit distance K-gram = A sequence of k characters used in indexing</p> Signup and view all the answers

    Which of the following operations is NOT one of the basic operations in edit distance?

    <p>Copy</p> Signup and view all the answers

    The primary function of spelling correction systems is to retrieve documents indexed by the misspelled word.

    <p>False</p> Signup and view all the answers

    What are two alternatives mentioned for isolated word correction?

    <p>Edit distance and K-gram overlap</p> Signup and view all the answers

    What does the Jaccard coefficient measure?

    <p>The ratio of the intersection to the union of two sets</p> Signup and view all the answers

    The Jaccard coefficient can only be used with sets of the same size.

    <p>False</p> Signup and view all the answers

    What threshold value would indicate a match using the Jaccard coefficient in this context?

    <p>greater than 0.8</p> Signup and view all the answers

    The process of identifying overlapping elements in k-grams is called __________.

    <p>k-gram overlap</p> Signup and view all the answers

    Match the bigram sets to their respective words from the given entries:

    <p>lorm = lo, or, rm alone = al, lo, on, ne lord = lo, or, rd sloth = sl, lo, ot, th</p> Signup and view all the answers

    Which of the following statements is true about ranked retrieval?

    <p>It ranks documents based on their relevance to the query.</p> Signup and view all the answers

    In a k-gram index, overlapping k-grams must be the same length.

    <p>False</p> Signup and view all the answers

    What is the Jaccard coefficient for the terms 'lorm' and 'lord'?

    <p>0.5</p> Signup and view all the answers

    What does the term frequency (tf) refer to?

    <p>The number of times a term occurs in a document</p> Signup and view all the answers

    Document frequency (df) is used to assign high weights to common terms.

    <p>False</p> Signup and view all the answers

    What does tf-idf stand for?

    <p>term frequency-inverse document frequency</p> Signup and view all the answers

    The idf for a term is calculated as log(n / df_t), where n is the total number of documents and df_t is the ________.

    <p>document frequency of the term</p> Signup and view all the answers

    Match the term with its definition:

    <p>Term Frequency (tf) = Number of times a term occurs in a document Document Frequency (df) = Number of documents that contain a term Inverse Document Frequency (idf) = Measure of informativeness based on term rarity tf-idf = Combines term frequency and inverse document frequency</p> Signup and view all the answers

    When is the tf-idf score highest?

    <p>When a term occurs frequently in a single document</p> Signup and view all the answers

    The higher the document frequency, the lower the discriminating power of a term.

    <p>True</p> Signup and view all the answers

    What is the primary goal of using term weighting in document queries?

    <p>To assess the relevance of documents to a query more accurately.</p> Signup and view all the answers

    What formula is used to compute the score for a document based on a query?

    <p>Score(q,d) = ∑ tf.idf, d</p> Signup and view all the answers

    The tf-idf weight for 'Caesar' in document 3 is greater than zero.

    <p>False</p> Signup and view all the answers

    Calculate the tf-idf for 'mercy' in document 1.

    <p>79.2</p> Signup and view all the answers

    The measure of __________ is the ratio of relevant records retrieved to all retrieved records.

    <p>precision</p> Signup and view all the answers

    Match the term with its corresponding tf-idf value in document 1:

    <p>Brutus = 5.1 Caesar = 21 mercy = 79.2</p> Signup and view all the answers

    What is the idf value of 'mercy'?

    <p>2.4</p> Signup and view all the answers

    Recall measures the number of relevant records retrieved compared to the total number of irrelevant records.

    <p>False</p> Signup and view all the answers

    What is the formula for recall?

    <p>Recall = (relevant records retrieved) / (total relevant records)</p> Signup and view all the answers

    Study Notes

    Web Engineering & Development Lecture 5: Search Engines (Tolerance Retrieval)

    • The lecture focuses on search engines and tolerance retrieval.
    • The agenda covers wild-card queries (permuterm index, k-gram index, spelling correction, edit distance, k-gram overlap), ranking documents (TF-IDF weights), evaluating search engines (precision and recall).
    • Wildcard queries use the * symbol to represent any character(s).
    • A wildcard query like aeiou seeks documents containing terms with all five vowels in order.
    • Permuterm indexing is a technique to handle wildcard queries by rotating the query to have the wildcard at the end of the string.
    • K-gram indexing enumerates all k-grams (sequences of k characters) in a term.
    • Bigrams (2-grams) are an example of k-grams , e.g. “April is the cruelest month”
    • A k-gram index maps k-grams to the terms that contain them.
    • Spelling correction mechanisms can either retrieve docs with the correct spelling or return suggested alternatives. Techniques include edit distance and k-gram overlap.
    • Edit distance calculates the minimum number of operations (insert, delete, replace) needed to transform one string into another.
    • The Levenshtein distance is a specific edit distance algorithm, represented as a matrix.
    • K-gram overlap retrieves terms with many matching k-grams to the query
    • The Jaccard coefficient is a measure for k-gram overlap. It compares two sets of k-grams, considering shared k-grams compared to the total number of k-grams.
    • Weighted terms (tf-idf) are used to boost relevant documents.
    • Term frequency (tf) is a term's frequency in a document.
    • Inverse Document Frequency (idf) is a measure of a term's informativeness (its rarity).
    • TF-IDF combines tf and idf.
    • A higher tf-idf score suggests a stronger match between a document and a query.
    • Search effectiveness is measured using precision and recall.
    • Retrieval effectiveness is measured by the recall and precision of retrieved documents.
    • Precision assesses the proportion of retrieved documents that are relevant to the query
    • Recall assesses the proportion of all relevant documents in the database that were retrieved.

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    Description

    This lecture delves into the intricacies of search engines, focusing on tolerance retrieval techniques such as wild-card queries and permuterm indexes. It also covers the essential aspects of document ranking, precision, and recall evaluation methods. Gain insights into how k-grams and spelling correction mechanisms enhance search engine performance.

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