Information retrieval : Modelling

introduction Modelling

•IR systems usually adopt index terms to process queries
•Index term:
–a keyword or group of selected words
–any word (more general)
•Stemming might be used:
–connect: connecting, connection, connections
•An inverted file is built for the chosen index terms

•Matching at index term level is quite imprecise
•No surprise that users get frequently unsatisfied
•Since most users have no training in query formation, problem is even worst
•Frequent dissatisfaction of Web users
•Issue of deciding relevance is critical for IR systems: ranking

•A ranking is an ordering of the documents retrieved that (hopefully) reflects the relevance of the documents to the user query
•A ranking is based on fundamental premisses regarding the notion of relevance, such as:
–common sets of index terms
–sharing of weighted terms
–likelihood of relevance
•Each set of premisses leads to a distinct IR model

IR : MODELS

Retrieval: Ad Hoc x Filtering

Classic IR Models - Basic Concepts

•Each document represented by a set of representative keywords or index terms
•An index term is a document word useful for remembering the document main themes
•Usually, index terms are nouns because nouns have meaning by themselves
•However, search engines assume that all words are index terms (full text representation)

•Not all terms are equally useful for representing the document contents: less frequent terms allow identifying a narrower set of documents
•The importance of the index terms is represented by weights associated to them
•Let
–ki be an index term
–dj be a document
–wij is a weight associated with (ki,dj)
•The weight wij quantifies the importance of the index term for describing the document contents

–Ki is an index term
–dj is a document
–t is the total number of docs
–K = (k1, k2, …, kt) is the set of all index terms
–wij >= 0 is a weight associated with (ki,dj)
–wij = 0 indicates that term does not belong to doc
–vec(dj) = (w1j, w2j, …, wtj) is a weighted vector associated with the document dj
–gi(vec(dj)) = wij is a function which returns the weight associated with pair (ki,dj)

The Boolean Model

•Simple model based on set theory
•Queries specified as boolean expressions
–precise semantics
–neat formalism
–q = ka Ù (kb Ú Økc)
•Terms are either present or absent. Thus, wij e {0,1}


Drawbacks of the Boolean Model

•Retrieval based on binary decision criteria with no notion of partial matching
•No ranking of the documents is provided (absence of a grading scale)
•Information need has to be translated into a Boolean expression which most users find awkward
•The Boolean queries formulated by the users are most often too simplistic
•As a consequence, the Boolean model frequently returns either too few or too many documents in response to a user query

The Vector Model

•Use of binary weights is too limiting
•Non-binary weights provide consideration for partial matches
•These term weights are used to compute a degree of similarity between a query and each document
•Ranked set of documents provides for better matching

•Define:
–wij > 0 whenever ki Î dj
–wiq >= 0 associated with the pair (ki,q)
– vec(dj) = (w1j, w2j, ..., wtj) vec(q) = (w1q, w2q, ..., wtq)
–To each term ki is associated a unitary vector vec(i)
–The unitary vectors vec(i) and vec(j) are assumed to be orthonormal (i.e., index terms are assumed to occur independently within the documents)
•The t unitary vectors vec(i) form an orthonormal basis for a t-dimensional space
•In this space, queries and documents are represented as weighted vectors


•Sim(q,dj) = [S wij * wiq] / dj * q
•How to compute the weights wij and wiq ?
•A good weight must take into account two effects:
–quantification of intra-document contents (similarity)
•tf factor, the term frequency within a document
–quantification of inter-documents separation (dissi-milarity)
•idf factor, the inverse document frequency
–wij = tf(i,j) * idf(i) ß unNormalized

•Let,
–N be the total number of docs in the collection
–ni be the number of docs which contain ki
–freq(i,j) raw frequency of ki within dj
•A normalized tf factor is given by
–f(i,j) = freq(i,j) / max(freq(l,j))
–where the maximum is computed over all terms which occur within the document dj
•The idf factor is computed as
–idf(i) = log (N/ni)
–the log is used to make the values of tf and idf comparable. It can also be interpreted as the amount of information associated with the term ki.

•The best term-weighting schemes use weights which are give by
–wij = f(i,j) * idf(i)
= freq(i,j) / max(freq(l,j)) * log(N/ni)
the strategy is called a tf-idf weighting scheme (Normalized)
•For the query term weights, a suggestion is
–wiq = (0.5 + [0.5 * freq(i,q) / max(freq(l,q)]) * log(N/ni)
= (0.5 + 0.5 * f(i,q)) * idf(i)
•The vector model with tf-idf weights is a good ranking strategy with general collections
•The vector model is usually as good as the known ranking alternatives. It is also simple and fast to compute.

•Advantages:
–term-weighting improves quality of the answer set
–partial matching allows retrieval of docs that approximate the query conditions
–cosine ranking formula sorts documents according to degree of similarity to the query
•Disadvantages:
–assumes independence of index terms (??); not clear that this is bad though