Program visualization (PV) is one of the approaches to assist novices in introductory programming courses. We investigated the effect of a newly developed PV tool to teach tracing to first-year programming students at a South-African university. The tool takes the form of an interactive computer-based tutorial that teaches students how to draw variable diagrams (VD). All students registered for the introductory C++ module, received the tutorial with their study material and could use it to help answer assignment questions. To determine the effect of using the tutorial, students did an assignment for which they could use the tutorial and completed a questionnaire. Through the questionnaire we acquired biographical data, found out how students used the tutorial and also how they experienced using it. We then correlated these data statistically with their assignment marks. We found that time spent using the tutorial, programming experience and certain biographical properties contributed to higher marks. Drawing their own VDs to understand and debug programs also contributed to better marks.