Fig. 2

Item responses on the PHQ-9. a The item responses of the all items showed a common pattern of different types of distributions with a boundary at “several days.” Between “not at all” and “several days,” as indicated by the red arrow, the lines for the nine items crossed at a single point. Between “several days” and “nearly every day,” lines for the nine items seemed to barely cross each other. As indicated by the black arrow, the line for item 9 followed the same pattern as those of other depressive symptom items. b Using a log-normal scale, lines of the item responses showed a parallel pattern from “several days” to “nearly every day.” The gradient of the linear pattern of item responses moved downward between “several days” and “more than half the days” (blue arrow) and horizontally between “more than half the days” and “nearly every day” (red arrow). As indicated by the black arrow, although the line for item 9 was a little away from other lines, it was almost parallel to those of other depressive symptom items