Before revision: As pointed out by Kahneman in Thinking, Fast and Slow (p. 252), one of the reasons reference class forecasting is effective for debasing is that, in contrast to conventional forecasting methods, it takes into account the so-called "unknown unknowns."
After revision: As pointed out by Kahneman in Thinking, Fast and Slow (p. 252), one of the reasons reference class forecasting is effective for debiasing is that, in contrast to conventional forecasting methods, it takes into account the so-called "unknown unknowns."