The grade is a number assigned to every event based on which pixels in its 3x3 island are above their threshold value. The initial grade is assigned by on-board processing, which first finds local maxima, then analyzes the values of the surrounding 3x3 neighboring pixels. Based on this pattern, the event is assigned a grade. For example, a single-pixel event has a grade of 0. See the Event Grade section of the Proposers' Observatory Guide for more information.