Although the idiographic approach to studying personality might give a very rich and detailed accounting of a person's life, the findings are often based on post-hoc explanations of events that have already occurred. this makes the findings difficult to contest. this is a problem for which principle of critical thinking?

I believe the answer is:  Falsifiability
 Falsifiability refers to the probability of a statement or a theory to be false.
In studying personality, the factors that could influence the result of the study are abundant, ranging from external factors, interaction with other people, the book we read, genetic factors, etc. Because of this, the rate of falsifiability would tend to be higher.