These are the unknowns in the primal model that determine the importance attributed to each factor. Since the value assigned to each weight depends on the measurement scale of the factor itself, it is difficult to compare the weights from different factors.

A factor which consists of a set of predefined values that are ranked in a specific order. An ordinal variable can be seen as a special case of a categorical variable. If the values of an ordinal factor need to be scaled, the inherent order must be preserved (Cook et al 1993). [Cook W, Kress M and Seil ord L (1993), ‘On the use of ordinal data in data envelopment analysis’, J. of the Opi. Res. Soc., 44, pp 133-140.]

A unit is said to be scale efficient when its size of operations is optimal so that any modifications on its size will render the unit less efficient. The value for scale efficiency is obtained by dividing the aggregate efficiency by the technical efficiency.

If a DMU is rated efficient and more or less equal importance has been given to each of its factors, it is said to exhibit well-rounded performance.

Generally taken to mean an odd unit with an input-output mix that significantly differs from the rest of the units. This can be due to a measurement error, or as a result of the outlier having different operating practices. Outliers that are efficient can introduce bias into the analysis. See Epstein and Henderson (1989). [Epstein M K and Henderson J C (1989) ‘Data envelopment analysis for managerial control and diagnosis’, Decision Sciences, 20, pp 90-119.]

The so-called ‘size’ of the DMU, it may be associated with either the inputs or the outputs, depending on the situation.

Tabular method for examining the changes in the efficiencies of a set of units over time. A set of time periods (1 ..t) is chosen and the efficiency of each unit (1..n) is computed separately for each period so that the efficiency of a given unit over each period is treated as a new unit resulting in a total number of tn units.

A factor which describes the amount of goods, services or other outcome obtained as a result of the processing of resources. Also, any factor which describes the qualitative nature of the resulting outcome.

The additional improvement (increase in outputs and/or decrease in inputs) needed for a unit to become efficient.

In an output-oriented model, an inefficient unit is made efficient through the proportional increase of its outputs, while the inputs proportions remain unchanged.