Open-ended tasks are common in Science, Technology, Engineering and Mathematics (STEM) education. However, as far as we know, no tools have been developed to assist in the assessment of the solution process of open-ended questions. In this paper, we propose the use of analysis of traces as a tool to address this need. To illustrate this approach, we developed a modified version of R Commander that collects traces of students' actions and described a way to analyze them by using regular expressions. We used this tool in an undergraduate introductory statistics course. The traces were analyzed by comparing them to predefined problem-solving steps, arranged by the instructor. The analyses provide information about the time students spent on the activity, their work intensity and the choices they made when solving open-ended questions. This automated assessment tool provides grades highly correlated to those obtained by a traditional test and traditional grading scheme.