Recall (recall ratio)

Recall is one among other measures of the performance of an information system. It was used for the first time in the so-called "Cranfield II experiments" (Cleverdon et al., 1966). Recall is defined as the proportion between retrieved relevant documents and the total number of relevant documents in the system in question.

 

 

Recall = a : (a + c) X 100%, where
a = number of retrieved, relevant documents,
c = number of non-retrieved, relevant documents (sometimes termed "silence").



Recall is thus an expression of how exhaustive a search for documents is. Recall constitutes together with precision the most common measures (and concepts) for the measurements of the performance of retrieval systems.

In information retrieval exists a long range of strategies for the increase of recall. Among those are searching in both "natural language fields" as in controlled vocabulary fields, the inclusion of synonyms (in natural language) in the query, inclusion of broader terms, restricted use of "and" in Boolean logic etc. Such strategies are, however seldom without costs but have a tendency to lower precision.
 

Recall is difficult to measure on operational systems, why test collections have to be used in research.

 

"Total recall" is a demand in certain kinds of searches, for example patent applications and search for side effects in drugs.

"Example 1: A physician has considered a certain drug for the treatment of a patient. In order to find out more about the exact effect of the drug for the disease of the patient, he requests a literature search. All of the documents retrieved report favorably about the results of the drug considered. The     physician is very satisfied with the search results and proceeds to prescribe the drug. Unfortunately, another document was not retrieved and not examined by the physician. It reports on a case very similar to the patient's in which the drug had very serious side effects which outweighed its benefits. No harm done by missing that document?" (Soergel, 1976, p. 258 ).

 
 

Literature:

 

Cleverdon, C. W.; Mills, J. & Keen, E. M. (1966). Factors determining the performance of indexing systems. Vol. 1-2. Cranfield, U.K.: College of Aeronautics.

Soergel, D. (1976). Is user satisfaction a hobgoblin? Journal of the American Society for Information Science, 27(4), 256-259.

 

 

Birger Hjørland

Last edited: 16-03-2010

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