"The only reliable method is to conduct a forecasting tournament in which independent judges ask all participants to make the same forecasts in the same timeframes. And forecasts must be expressed numerically, so there can be no hiding behind vague verbiage. Words like "may" or "possible" can mean anything from probabilities as low as 0.001% to as high as 60% or 70%. But 80% always and only means 80%. In the late 1980s one of us (Philip Tetlock) launched such a tournament. It involved 284 economists, political scientists, intelligence analysts and journalists and collected almost 28,000 predictions. The results were startling." The Economist.