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Definition of likelihood ratio

 

The likelihood ratio for candidate j for a given source i is (de Ruiter et al. 1977 [2],Prestage & Peacock 1983 [4], Wolstencroft et al. 1986 [5],Irwin 1996 [7]):

 

with the general expressions

 

and

 

such that, as in [4]

 

where is defined as

 

In equation gif, may be which depends on the magnitude of the candidate.

Note that a number of authors (Benn 1983 [3], [5], Sutherland & Saunders 1992 [6]) have included the factor Q (a priori probability that an identification exists above the plate limit) in the expressions for and hence . How ever this factor Q is unknown and has to be guessed, which goes against the idea that a likelyhood ratio should be derived from the data and make no a priori assumptions. For this reason we chose no to include it, although it (or rather , which is very similar to it) appears in the definition of reliability (see gif