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.]