Allowable Value Fields

Input expressions are written in the cells of a Decision Table and are usually quite simple, being either:

• Values, such as 'Gold' or
• Tests, such as 'Age < 25'

The values or variables in these expressions must be provided through Input Data, which should be visible in the Decision Requirement diagram. The universe of available or possible input values or ranges will typically have been discussed at the definition level, and these form an all important reference for the Decision Table definition. For example, the decision for a loan application might rely on an input of Customer Level, which could be defined in the business as an enumeration of values - 'Gold, Silver, Bronze' - and could also depend on a simple Age value defined as '18 < Age < 65'. It is important when defining the rules and their logic within the table to include these constraints, as it will make the table easier to understand for humans and easier to validate for the tool. Without these being defined a modeler might assume the table is complete, but the validator could possibly prescribe otherwise.

Irrespective of how a modeler defines the expressions, the input values should be exclusive and complete to ensure the table is defined correctly and will produce the expected outputs at run time (after it is implemented and used in practice).

• Exclusive - means that the input values are disjoint; that is, they do not overlap with each other - no overlaps
• Complete - means all relevant input values defined in the domain have been captured - no gaps

These input constraints or restrictions can be added to the header just below both the input and output columns, in a table with rules oriented horizontally.

This screenshot shows that a modeler has defined the allowable values for the input columns for the Finance Application Decision Table. The risk assessment input is only allowed to have three values, namely High, Moderate and Low. A value such a Minimal, if listed in  the table, would result in a validation error. We will look at validation extensively in a subsequent section of the Guide, but this is an example output from the validation facility in the case where 'Minimal' is erroneously used, as we have discussed.