The Boolean model of information retrieval is a classical information retrieval model and, at the same time, the first and most-adopted one. It is used by many IR systems to this day. The BIR is based on Boolean logic and classical set theory in that both the documents to be searched and the user's query are conceived as sets of terms. Retrieval is based on whether or not the documents contain the query terms.
Definitions
An index term is a word or expression, which may be stemmed, describing or characterizing a document, such as a keyword given for a journal article. Letbe the set of all such index terms. A document is any subset of. Letbe the set of all documents. A query is a Boolean expression in normal form:where is true for when. We seek to find the set of documents that satisfy. This operation is called retrieval and consists of the following two steps:
Example
Let the set of original documents be, for example where = "Bayes' principle: The principle that, in estimating a parameter, one should initially assume that each possible value has equal probability." = "Bayesian decision theory: A mathematical theory of decision-making which presumes utility and probability functions, and according to which the act to be chosen is the Bayes act, i.e. the one with highest subjective expected utility. If one had unlimited time and calculating power with which to make every decision, this procedure would be the best way to make any decision." = "Bayesian epistemology: A philosophical theory which holds that the epistemic status of a proposition is best measured by a probability and that the proper way to revise this probability is given by Bayesian conditionalisation or similar procedures. A Bayesian epistemologist would use probability to define, and explore the relationship between, concepts such as epistemic status, support or explanatory power." Let the set of terms be: Then, the set of documents is as follows: where Let the query be: Then to retrieve the relevant documents:
Firstly, the following sets and of documents are obtained :
Finally, the following documents are retrieved in response to
This means that the original document is the answer to. Obviously, if there is more than one document with the same representation, every such document is retrieved. Such documents are indistinguishable in the BIR.
Advantages
Clean formalism
Easy to implement
Intuitive concept
Disadvantages
Exact matching may retrieve too few or too many documents
Hard to translate a query into a Boolean expression
All terms are equally weighted
More like data retrieval than information retrieval
From a pure formal mathematical point of view, the BIR is straightforward. From a practical point of view, however, several further problems should be solved that relate to algorithms and data structures, such as, for example, the choice of terms, stemming, hash tables, inverted file structure, and so on.
Hash sets
Another possibility is to use hash sets. Each document is represented by a hash table which contains every single term of that document. Since hash table size increases and decreases in real time with the addition and removal of terms, each document will occupy much less space in memory. However, it will have a slowdown in performance because the operations are more complex than with bit vectors. On the worst-case performance can degrade from O to O. On the average case, the performance slowdown will not be that much worse than bit vectors and the space usage is much more efficient.
