Humans, of course, are not the only source of uncertainty in predictions. Last December's European storm shows how capricious nature can be. Similarly, scientists have been trying for decades to predict earthquakes, with little or no success. In the late 1980s, some of the world's best earthquake scientists predicted, with ninety-five percent certainty, that a major earthquake would strike a segment of the San Andreas Fault south of San Francisco within five years. The earthquake didn't occur.
Experience is the best guide.
Every day, we read the weather forecast, and see the probabilities assigned to predictions of future events-eighty percent chance of rain, etc. The National Weather Service issues about 10 million weather forecasts each year, and this gives them lots of information that they can use to see how well they are doing. Of equal importance, it gives the public lots of opportunities to evaluate the predictions as well, and develop expectations that can guide actions. No one expects weather predictions to be perfect, and one's experience with past predictions allows one to make decisions in response to new predictions. For example, during the muggy summer months in Washington, DC, weather forecasts routinely show a chance of thunderstorms in the afternoon, but experience quickly teaches that this is not sufficient reason to cancel a picnic.