Mount Katahdin (in Maine) is at the end of the Appalachian Trail (well, if you start in Georgia for a northern thru hike - which is the most common approach).
If I sit at the top of Mount Katahdin and keep track of all the hikers passing through, asking them where they have come from and noting how many started in Georgia and completed the entire 2180 miles I will get the impression that hiking the AT from Georgia is not a big deal. Let’s say in a season I note 1000 people complete the full south to north AT thru hike. I will have the impression that everyone that starts finishes because all I am observing are the finishers. My conclusion is biased. I have only observed the finishers. I have not observed, bounded from my place in reality I have not observed all the people that started and stopped along the way.
If you are a PT working in an acute care hospital and only see people that survive otherwise life threatening illnesses and you do not consider the inherent biases associated with such observations you run the risk of drawing incorrect (biased) conclusions…..just like the PT that figured complete AV heart block was non life threatening because they see people that survived it all the time. What they do not see are those that did not survive…….and for many that do not survive we do not even know that complete AV heart block was the cause of “sudden cardiac death” as a rather vague mechanistic explanation.