European Centre for Medium Range Weather Forecasts
Essential Climate Variables
European Reanalysis of Global Climate Observations -2
The unknown difference between the measured value and the (unknown) true value. The error is a specific draw from the probability distribution function described by the uncertainty.
Expanded uncertainty is the standard uncertainty multiplied by a coverage factor, k. The coverage factor is chosen to obtain a desired level of confidence. Most commonly a 95 % confidence interval is chosen. For a Gaussian distribution this is achieved with a coverage factor k = 2. (Note that strictly this provides a 95.45 % confidence interval).