A
rule of thumb that it is safe to make
generalizations in an
argument if a given
condition is
true ninety percent of the
time or more.
One must take care to not also assume the exception never happens, however, especially when the exception is drastic and inevitable over the long run. i.e., it is very unlikely for an extremely large meteor to hit the Earth at any given moment, but it is almost certain that one will hit eventually.