The Working Group 5 of the International Medical Informatics Associa tion (IMIA) is dedicated to information systems in primary care with special emphasis on computer systems in the doctor's office. Accord ingly, a conference was held in Hannover in 1980 where the first ap proaches were described and experiences in system analysis, system construction and evaluation were discussed (Rienhoff, O. and Abrams, M.E. (eds.): The Computer in the Doctor's Office, North-Holland, Amsterdam: 1980). Computer hardware and software development has been rapid over the last years. Thus the prerequisites for a successful support of the work of the physician in his office have improved. But system con struction still lags behind and the actual penetration of systems is lower than 2 % of the doctors' offices in most countries. This applies to industrialized countries. However, attempts are made everywhere to improve primary care by means of modern information technology. Information systems depend upon the real environment into which they are placed: administrative procedures govern priorities and procedures in doctor's office computers, possibilities to defray cost upon the various carriers or the patients have a great influence on the propagation of systems. Furthermore, various procedures of accounting or re-imbursement may lead to a facilitation or to a delay of the in troduction of systems. The 'art of medical practice' has reached a comparable standard within at least the industrialized countries."

The Working Group 5 of the International Medical Informatics Associ ation (IMIA) dedicates its work to information systems in primary and ambulatory care. The first conference of this Working Group in Hanno ver in 1980 produced a review of the state of the art of that time and gave perspectives for future development (Rienhoff, O. and Abrams, M.E. (eds.): The Computer in the Doctor's Office, Horth Holland, Amsterdam: 1980). In the meantime, a rapid development has taken place. Therefore it seemed appropriate to hold another working conference which was con ducted in Munich, December 2-6, 1985. The goal of this working conference was to review the developments in this field and to critically evaluate the progress achieved so far. The conference addressed general principles in system development and prerequisists for their successful introduction into routine use. Furthermore, the topics of expert systems and new technologies were discussed in the context of their usefulness and usability in ambula tory care systems. As before, it became very clear that actual computer systems to sup port ambulatory and/or primary care have to meet the conditions of the health care delivery systems, into which they are to be placed."

The current scarcity of expert systems where the reasoning is based on Bayesian probability theory may be due to misconceptions about probabilities found in the literature. As argued by Cheeseman (1985), these misconceptions have led to the attitude: "The Bayesian approach doesn't work - so here is a new scheme." Several of these expert systems based on ad hoc "probability" concepts have been successful in a number of ways, demonstrating the necessity of being able to handle uncertainty in medical expert systems. They also demonstrate the need for a theoretically sound handling of uncertainty. In Andersen et al. (1986) it was postulated that knowledge organized in a causal network can be used for a unified approach to the main tasks of a medical expert system: diagnosis, planning of tests and explanations. The present paper explores this postulate in a causal probabilistic network. It also provides a practical demonstration that the problems supposedly associated with probabilistic networks are either non-existent or that practical solutions can be found. This paper reports on the methods implemented in MUNIN* -an expert system for electromyography (EMG) (Andreassen et al. 1987). EMG is the diagnosis of muscle and nerve diseases through analysis of bioelectrical signals from muscle and nerve tissue. In Andreassen et al.