Probability or frequency? Expressing forecast uncertainty in public weather forecasts

Is uncertainty expressed as frequency easier for non-experts to understand than uncertainty expressed its probability? The experiment reported here compared participants' responses to the same wind speed forecast expressed several different ways. Three different uncertainty expressions were tested (90%, 9 times in 10, or 90 out of 100%). Also tested was whether understanding was improved by including a short phrase explaining, in lay terms, how the forecast wits derived (adding a reference class). Results suggested that, contrary to prior research, participants better understood the forecast when it was presented in a probability format rather than a frequency format. Furthermore, specifying a reference class did not facilitate understanding. Copyright (C) 2009 Royal Meteorological Society