Errors, or "bugs", in addition and subtraction problems of signed numbers are driven by misconceptions or incomplete knowledge at various components of procedural tasks, and result in erroneous rules of operation for carrying out computational problems. These rules, including the right one, are represented by the responses to a particular set of items because each rule generates a unique set of responses to the items. This study attempts to develop a method for quantifying these bugs according to the seriousness of the mistakes. This is done by referring to the procedural network, or task tree. If a role is a result of a misconception of an earlier level in the network, then it is more likely committed by students who are in the early stage of learning, or by lower ability students. The Procedural Steps Conformity Index was designed to quantitatively express these errors. (Author/BW)