Half Empty, Half Full - An Examination of Subtractive Versus Additive Assessment

Abstract

In this paper, we present a case study comparing subtractive marking schemes with the more common technique of additive marking. Although often accepted for use in oral language and multiple choice exams, subtractive marking has yet to see serious consideration as an accepted technique for assessing other more common paper exams, particularly Computer Science exams with subjective questions. This paper is presented in a number of sections. After a brief overview of previous investigations in the area, we outline an experiment conducted with real examination papers taken by students at Griffith College Dublin. In this experiment, we mark the same set of papers using both additive and subtractive schemes. We then summarise the differences between the two techniques and identify some of the challenges, advantages and disadvantages of subtractive approaches and also the motivations behind them. We also examine how different types of exam questions affect the difference between additive and subtractive marking and make the argument in favour of subtractive marking as a useful QA technique. Finally, we present the results of a student survey regarding their opinions on subtractive marking in order to gain an insight as to how students feel about the concept and what types of students feel strongly for or against it