A term used to denote a time series of non-negative integer values where some values are zero. For example, shipments to a store may be zero in some periods because a store’s inventory is too large. In this case, the demand is not zero, but it would appear to be so from the data. Croston’s method (Croston 1972) was proposed for this situation. It contains an error that was corrected by Rao (1973). Willemain et al. (1994) provide evidence favorable to Croston’s method. Other procedures such as aggregating over time can also be used to solve the problem. See Armstrong (2001c).