“HOW PUNDITS NEVER GET IT WRONG”—THE 40% RULE. Rolfe Winkler and Justin Lahart had an article in the Wall Street Journal (February 27, 2018) about what they call the “40% rule” for predictions. They call the rule “a favorite forecasting tactic of Wall Street analysts….” Other examples: Tony Blair says there’s a 40% chance that Brexit will be reversed; a well-known economist says there’s a 40% chance that NAFTA will be terminated. The article cites Philip Tetlock, an expert on forecasting, who objects to the use of terms like “distinct possibility” rather than probabilities, which provide a little less flexibility—in part because nobody knows what probability corresponds to a “distinct possibility”.
Here is one many posts on the blog (again, the archive feature will locate them) about how forecasts should be expressed in numbers rather than words. I had not seen any reference to the “40% rule” previously.