An interview investigation was undertaken in which a group of nurses and a group of technicians reasoned about evidence for the existence of a causal relation in a medical and in a technical case. A statement about the existence of the relation was presented and the subjects were then asked as to what evidence there might and should be behind such a statement. In an unstructured in-depth interview, they outlined their conceptions of the problem of evidence in each example by suggesting the kinds of investigations that would be relevant. The nurses and technicians relied on their different knowledge of the specific subject-matter and they placed the evidence problem in different contexts giving different meanings to the problem. (Author)