The paper presents a novel research methodology, called the Triadic Method to highlight users? personal sense making process. The method is drawn from the work of Norbert Elias (1991), George Kelly (1963) and Max Weber (1897). To illustrate the relevance of the Triadic Method, the paper presents a case study of how an experienced user, here an air traffic controller, goes about using an Information System when resolving a problem in an urgent situation. The results of the study show that air controllers? decision making process nuances a strict interpretation of Herbert Simon's (1977) hypothesis of ?limited rationality?. An analysis of the data reveals that air traffic controllers' use IS related data to disconfirm a series of possible choices in a systematic elimination process when resolving an air traffic conflict. These findings have methodological implications for future development of IS, notably in a current multidisciplinary research project called SAMOSA, concerning the implementation of whole-body, millimetre wave imaging scanners in French airports.