Case is an event that took place in the past and can be an example for similar steps in present. Case-based reasoning (CBR) is a methodology of finding a decision based on previous experience. The main difficulties in CBR are recognition of present situation of decision-making and finding similar case in the past. Making a decision in the field of purchasing drugs is a quite difficult and many-valued task with a plenty of various parameters, which however might be solved with the help of CBR. CBR is an important and perspective area of artificial intelligence (AI) decision support systems. Constantly growing research amount has already led to breakthroughs in theoretical part of AI and in AI-based applications. The general case model and formula for determining cases similarity is suggested in this work. Application of the formula will help determine similarity of cases and make the better decision.

Keywords: case, case-based reasoning, case description, decision support system, case-based decision making