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| 149 |
A large number of measures of effectiveness can be derived from this table
......There is a functional relationship between all three involving a parameter called generality G which is a measure of the density of relevant documents in the collection
...For each request submitted to a retrieval system one of these tables can be constructed
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| 150 |
value can be calculated
...For a derivation of this relation from Bayes Theorem,the reader should consult the author s recent paper on retrieval effectiveness [10]...Averaging techniques The method of pooling or averaging of the individual P R curves seems to have depended largely on the retrieval strategy employed
...where As is the set of documents relevant to request s
...where B [[lambda]]s is the set of documents retrieved at or above the co ordination level [[lambda]]...Figure 7
...An alternative approach to averaging is macro evaluation which can be independent of any parameter such as co ordination level
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| 165 |
In an analogous manner normalised precision is worked out
...The calculation of the areas is a bit more messy but simple to do see Salton [23],page 298
...The log function appears as a result of approximating [[Sigma]]1 r by its continuous analogue [[integral]]1 r dr,which is log r constant
...The area between the worst and best case is obtained in the same way as before using the same substitution,and is:The explicit form,with appropriate normalisation,for normalised precision is therefore:Once again it varies between 0 worst and 1 best
...A few comments about these measures are now in order
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