Introduction

Average probabilities are first mentioned here. They occur elsewhere in this book, in particular in discussions relating to discriminating power. They have a place in the assessment of the worth of a general area of evidence, such as head hairs, where frequency data are unavailable. However, it is very misleading to quote average probabilities in the courts as a measure of the value of the evidence. Other topics discussed include the effects of selection on the representativeness of samples and the assessment of rare events or events which may happen but have not been observed in the experience of the investigating scientist.