START is a system of primary triage performed on casualties at the scene of a mass-casualty incident. START is an acronym for "Simple Triage and Rapid Treatment" by which casualties are sorted into four triage categories. After conducting the triage simulation exercises, the authors realized that some students repeated the same errors despite having completed the triage classes. The purpose of this study is retrospectively to examine some mis-categorized cases to identify causes of triage errors, and thereby inform the future design of courses for triage education.Subjects were 115 fourth-year nursing students at a university in Kyushu, Japan who completed a two-day disaster nursing course. Nursing students were asked to fill out an answer sheet in the first session (hereafter referred to as "pre-intervention") and again in the second session held a week later (hereafter referred to as "post-intervention"). Many of those were about mis-categorizing "green" casualties as "yellow" and vice versa, which implied that students had difficulty in making triage decisions between the "walking wounded/minor" and "delayed" categories.The results of the study showed that most students were likely to answer the triage questions largely based on their perception of visual information (such as video images) rather than on triage criteria.