Traditionally, failure modes have been graphed after criticality has been assessed (rather than to determine criticality as in Palady’s area charts above) in what are known as criticality matrices. Although the axes of a criticality matrix are user-definable, the X-axis is usually based on severity, and the Y-axis is usually based on the probability of occurrence (which are occurrence ranking values when analysis is qualitative and calculated probability values when analysis is quantitative). Report versions of criticality matrices list individual items by identifiers that fall under each criticality rank.