The customary way to determine whether an adopted Supplemental Instruction (SI) program has been successful or not is by comparing course results for two groups, SI attendees and non-attendees. The division of SI attendees and non-attendees is generally done rather arbitrarily by prescribing a minimum number of SI sessions a student has to attend to be considered an SI attendee. Although the SI attendee vs. non-attendee concept is powerful in some respects, it tends to cloud the benefit of attending SI sessions. That a higher SI attendance leads to better course results is perhaps taken for granted, but in the few further studies that have been made, the picture of SI attendance rates vs. course results is not overly clear. The present study aims to contribute to how the degree of SI attendance affects course results in an engineering context at a Swedish University. In the study we divide the students into four categories, those with high, average, low, and no SI attendance. In terms of student success in a course, it is found that there is a clear relation between the number of SI sessions attended and course success. Students with high SI attendance do best followed by students with average, low, and no SI attendance, respectively.