Recently there was commentary in a forum that suggested the Pulmonary Embolism Rule-Out Criteria (PERC Rule) was essentially useless for detecting a pulmonary embolism (PE).  It started with an anecdote, which is a logical fallacy (post hoc ergo propter hoc) and went wild from there.  This led to the realization that many still do not understand how to use the Wells' Criteria for Pulmonary Embolism (referred to from here simply as the Wells' Criteria) and the PERC Rule.