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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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Unfortunately the ranking generated by a matching function is rarely a simple ordering,but more commonly a weak ordering
...For example,consider the weak ordering in Figure 7
...depending on how many non relevant documents precede the sixth relevant document
...4 10
...The above procedure leads immediately to a convenient intuitive derivation of a formula for the expected search length
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