For some predictands, such as precipitation, it does not make sense to predict negative values. However, the fact that zero is an absolute lower limit is not considered in the statistical procedures CPT uses. The zero-bound option automatically resets all non-negative predictions (including those for the confidence limits) to zero if they are negative. If there are any negative values in the input data, these are identified and an error message is given. If the anomalies or standardised anomalies option is switched on the zero-bound option will still apply, but only at the point where absolute zero is defined in terms of anomalies. (For example, if the climatological mean is 50, and the anomalies option is activated, anomalies of less than -50 will be reset to -50).