 Difficulties in assessing probabilities go hand in hand with the choice of the hypotheses of interest. To some approximation, hypotheses can be divided into three levels: offence, activity, and source level hypotheses. At the offence level, the issue is whether the defendant is guilty, as in the statement “Smith is guilty of manslaughter”. At the activity level, hypotheses describe what happened and what those involved did or did not do. An example of activity level hypothesis is “Smith stabbed the victim”. Finally, source level hypotheses describe the source of the traces, such as “The victim left the stains at the scene”, without specifying how the traces got there. Overlooking differences in hypothesis level can lead to serious confusions.