Abstract
Unexpected and unplanned events like lab errors can occur during scientific experiments or measurements. When they occur, a model that ignores them can be very inaccurate. Two diagnostic tools are compared. One is improb which equals the sum of the probabilities of all outcomes as likely as or less likely than the actual outcome. The second is a rarity index which is newer and equals the probability of the actual outcome divided by the probability of the most probable outcome. Serial dilution experiments are the motivating example and will be discussed extensively, although both methods may apply more generally.