Students' conceptions are characterized by some authors as having a high degree of coherence while, in the minds of others, they show little coherence and great heterogeneity. The objective of this study was to throw light on this problem by reference to "the particulate nature of matter," a topic where great discrepancies have been observed in the degree of coherence shown by students. We interviewed 43 students aged 9-22 years to evaluate the coherence of their replies, using a questionnaire in which different methodological strategies are incorporated, as will be explained below. A qualitative analysis of the students' replies to the interviewer's questions permitted a group of empirical categories to be established, which were then arranged hierarchically. The quantitative analysis of these categories gave rise to new categories, which we call structural categories. These permitted us to select the most significant variables, to identify the different types of conceptions held by the subjects and to assign to each a given level of conceptualization. They also revealed the high degree of coherence in the replies given to different tasks. The usefulness of the proposed method for studying students' conceptions and for evaluating their degree of coherence is confirmed.