"Internal" and "external" analysis

The basic MDS algorithm generates a configuration of points purely in accordance with the ordinal information in the data, i.e. the result is determined "internally" by the data matrix. This applies to all procedures, with the exception of PREFMAP, PROFIT, and PINDIS (which offers both internal and external analyses).

In some cases, the positions of some of the stimuli or variables may already be known or capable of being assumed. In this case an "external" analysis is performed, using the data to fit the remaining stimuli within this frame. A particularly important example occurs in preference mapping, where a system of preference judgements is related to a known, or hypothetical, configuration of stimulus points (see PREFMAP).