Errors that are not distributed symmetrically about the mean. This is common when trends are expressed in units (not percentages) and when there are large changes in the variable of interest. The forecaster might formulate the model with original data for a variety of reasons such as the presence of large measurement errors. As a result, forecast errors would tend to be skewed, such that they would be larger for cases when the actual (for the contrary series. For details, see Armstrong and Collopy (2001).