In previous papers in this journal [Cook R, Evett IW, Jackson G, Jones PJ a
nd Lambert JA. A model for case assessment and interpretation. Science & Ju
stice 1998; 38: 151-156. Cook R, Evett Mi, Jackson G, Jones PJ and Lambert
JA. A hierarchy of propositions: deciding which level to address in casewor
k. Science & Justice 1998; 38: 231-239], the authors have described a model
for case assessment and interpretation. This paper continues the discussio
n by studying the pre-assessment stage in more detail, using a hypothetical
fibres case in which there may be a two-way transfer. It is demonstrated t
hat a simple multiplication of likelihood ratios for the two directions of
transfer is not valid because of dependencies in the numerator. The solutio
n to this problem is described by means of a is made. Next the paper shows
how the pre-assessment is made. Next the paper shows how the pre-assessment
can be updated when a staged approach is taken: the results of the examina
tion of one of the garments are used to inform the decision about whether t
he second garment should be examined.