Growing interest in emotional interfaces and systems motivates the need for a standard and well-accepted way to measure emotional responses. We introduce the concept of Emotional Majority Agreement: a metric to characterize affective self-report instruments. Emotional majority agreement scores describe the extent to which a sample of participants agrees with regards to their emotional responses. We present a video-watching case study where 12 participants continuously rated how they felt using three self-report prototypes. Majority agreement measures differentiated videos that elicited more or less agreement, prototypes that supported more or less agreement, and individual differences between participants. We conclude with insights gained and future work.