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system so that if we were to adopt [[Delta]]as a measure of effectiveness we could be throwing away vital information needed to make an extrapolation to the performance of other systems
...The Cooper model expected search length In 1968,Cooper [20]stated:The primary function of a retrieval system is conceived to be that of saving its users to as great an extent as is possible,the labour of perusing and discarding irrelevant documents,in their search for relevant ones
...a only one relevant document is wanted;b some arbitrary number n is wanted;c all relevant documents are wanted;4 a given proportion of the relevant documents is wanted,etc
...Thus,the index is a measure of performance for a query of given type
...The output of a search strategy is assumed to be a weak ordering of documents
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The model We start by examining the structure which it is reasonable to assume for the measurement of effectiveness
...If R is the set of possible recall values and P is the set of possible precision values then we are interested in the set R x P with a relation on it
...Definition 1
...1 Connectedness:either e 1 >e 2 or e 2 >e 1 2 Transitivity:if e 1 >e 2 and e 2 >e 3 then e 1 >e 3 We insist that if two pairs can be ordered both ways then R 1,P 1 R 2,P 2,i
...We now turn to a second condition which is commonly called independence
...Definition 2
...All we are saying here is,given that at a constant recall precision we find a difference in effectiveness for two values of precision recall then this difference cannot be removed or reversed by changing the constant value
...We now come to a condition which is not quite as obvious as the preceding ones
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