4.2.1 Specification of criteria

   A criterion is defined by the variable of a  core model and the type. Criteria can be specified in a text file and/or can be defined interactively. A  core model definition typically uses meaningless names of variables, therefore criteria defined for  ISAAP should use  meaningful names (instead of using names of the core model variables as criteria names).

 An example of specification of criteria is provided in the file aez_dos.cri (its contents is shown in Table 1) that  predefines the criteria using the corresponding variables of the  AEZ  core model.

  

Table 1: Contents of the file with predefined criteria for the  AEZ core model.

The first word in a line contains the name of a variable (column) or of a constraint (row) of the  LP model. The second word must start with either v (to indicate that the name corresponds to a variable), or with c (for a constraint). Only the first letter of the second word is processed. The third word defines the name of  a criterion. The fourth word defines the type of  a criterion (one of: min, max or goal). The fifth word defines units in which the respective  criterion value is expressed.

 Criteria can also be specified interactively. In order to facilitate the selection of variables for medium-sized and large models, the definition of criteria is split into two stages:

• First, the user interactively selects from a list of variables those which will be used for criteria. The dialog for selecting variables has the help button which provides the necessary information. In order to avoid scrolling a long list of all variables of the  core model, one can define a mask (first characters of names of variables) for variables that are displayed for selection. Several masks can be defined sequentially for providing a list composed of names that start with different characters. For small models the user can define an empty mask, which will result in including all the variables into the selection list.
• Second, the user selects for each variable a criterion name and its type. The criterion name is limited to 6 characters and in order to conform to the requirements of some solvers have to be composed of printable characters which do not include blank spaces.

The current implementation of  MCMA and  ISAAP allows for three types of criteria:      minimized, maximized and goal. The meaning of the first two types is obvious, but the goal type requires an explanation.

The goal type of a criterion should be used only if the meaning of a criterion is such that a criterion should neither be minimized nor maximized, but it should attain a given goal  (target) value. In many situations this type is useful (for example for the analysis of soft equality constraints) and it has a clear interpretation. For the goal type of a criterion the distance between a given  target value (which can be changed during the interaction) and a criterion value is minimized. Note that the shape of a minimized or maximized criterion can be switched during the  MCMA to a stabilized criterion, which has exactly the same meaning as a goal type of a criterion. Afterwards, the status of  a stabilized criterion can be changed back to its original type, except for the goal type criteria that always have the status stabilized. However, a criterion type, once defined in the  preparatory stage of  MCMA, can not be changed. The distinction between the goal type and the stabilized status is forced by the need of assuring the consistency of  MCMA.

For a     goal type criterion the user has to interactively specify  an initial target value. The  Utopia point component for such a criterion is the minimal attainable deviation from this value. This value can be changed during  MCMA, therefore for such a criterion (contrary to other types of criteria) the  Utopia value can also be changed during  MCMA. This might be misleading for less experienced users.

Janusz Granat	- Institute of Control and Computation Engineering
Marek Makowski	- International Institute for Applied Systems Analysis