The initial version of the model, proposed by Estes (1972), is very different from current versions (e.g., Estes, 1997; Nairne, 1992). Currently, the perturbation model describes how encoded values can drift in multidimensional space. The presentation of an item leads to an initial representation of values on a number of dimensions such as position within a list and list-position within a series of lists. These encoded values lose precision due to perturbation, the graduate drifting away from the original values.