The *30 in 30 Briefing Series *focuses on a new cognitive bias, fallacy or heuristic in every single publication. Through this Briefing we want to provide you with a rough overview of the cognitive phenomena most likely to occur in the legal profession. Today’s content: The Base Rate Fallacy. Please be aware, that basic knowledge of Bayesian Rationality (see my short essay HERE) is helpful to understand the Base Rate Fallacy.

The Base Rate Fallacy is a formal fallacy. It occurs when too little weight is placed on the probability base rate of an event. People are succumbing to the base rate fallacy when erroneously judging the likelihood of an event without taking into account all the relevant data. The underlying mechanism is an overt focus on new information without acknowledging how this impacts the original assumptions. Old information is still evaluated and taken into account, however the newer pieces of information are given too much weight. The opposite effect is called the conservatism bias. This is the tendency where the base rate is overly rated as compared to more recent observations.

Let us try to understand this fallacy by looking at an example: Police officers utilize breathalyzers reporting false drunkenness in 5% of the cases in which the driver is sober. Yet, the breathalyzers do not ever fail to detect a truly drunk person. Every 1000th driver is driving drunk. Imagine police officers stopping one driver and asking him to take a breathalyzer test. It indicates that the driver is drunk. What is the probability he or she is really drunk? Many would answer as high as 0.95 but the correct probability is about 0.02.

Explanation: One driver is drunk and it is certain that his test result is correct. Therefore, there is one true positive test result. However, 999 drivers are not drunk while driving. Among these drivers there are 5 % false positive test results. Thus, there are 49.95 false positive test results. As a result, the probability that one of the drivers among the 1+49.95 positive test results really is drunk is 1 / 50.95= 0,196.

But how is this important to law and the legal profession? As Lawyers are confronted with facts every day they try to draw conclusions from them. However, they should be careful to evaluate the probability of every fact on its own. Whenever you are confronted with probabilities, you should keep the Base Rate Fallacy in mind. Focusing solely on old or new evidence will most likely lead to distorted results. With this fallacy occurring so often it will be very beneficial for you to detect and defuse it and become a more rational lawyer in crucial situations. For your client it will turn out highly beneficial.

**FLORIAN ZINK**