“[M]aking a prediction in terms of probability simply does not fit well with predicting an event that will happen only once.” A lawyer who tries to give a client the likelihood as to whether something will happen – for example, the odds that the Department of Justice will oppose a proposed merger – by stating a percentage (“75% chance this will get by”) is really saying that if this identical deal happened 100 times, for 25 of them the DOJ would step in and stop it. A good article in the ACC Docket, Nov. 2011 at 40, makes this point.
The authors recommend that lawyers use “natural frequencies” to express probabilities, such as “in 25 out of a 100 instances like this, anti-trust objections will kill the deal” (See my post of May 5, 2011: natural frequencies compared to percentages or decimals.). Otherwise, if the likelihood given the client is more than 50 percent, the client tends to remember that as a prediction that this time it will happen.