Seg3430A Tutorial 8

Decision Table

Logic Modeling

• Data flow diagrams do not show the logic inside the processes

• Logic modeling involves representing internal structure and functionality of processes depicted on a DFD

• Logic modeling can also be used to show when processes on a DFD occur

• For example, Structured English, Decision Tables, Decision Trees, State-transition diagrams, Sequence diagrams, Activity diagrams

Modeling Logic with Decision Tables

• Decision Table– A matrix representation of the logic of the decision– Specifies the possible conditions and the resulting

actions.– Best used for complicated decision logic

• Consists of three parts– Condition stubs: lists the conditions to the decision– Action stubs: lists the actions that result for a given set

of conditions– Rules: specifies which actions are to be followed for a

given set of conditions

Decision Table Methodology• Step 1: Identify conditions and values

– Find condition tests and all of the values• Step 2: Compute Max Number of Rules

– Multiply the number of values for each condition• Step 3: Identify possible actions

– Determine independent actions to be taken• Step 4: Enter all possible rules

– Fill in the values of the conditions in each numbered rule column• Step 5: Define actions for each Rule

– Mark the appropriate actions with X• Step 6: Verify the policy

– Review the decision table with end-users• Step 7: Simplify the table

– Eliminate and/or consolidate rules

An Example

• Scenario:– A marketing company wishes to construct a decision

table to decide how to treat clients according to three characteristics: Gender, City Dweller, and age group: A (under 30), B (between 30 and 60), C (over 60). The company has four products (W, X, Y and Z) to test market. Product W will appeal to female city dwellers. Product X will appeal to young females. Product Y will appeal to Male middle aged shoppers who do not live in cities. Product Z will appeal to all but not the older females.

An Example• Step 1: Three conditions &values:

– Gender F/M– City Y/N– Age group A/B/C

• Step 2:– Max. num. of rules = 2x2x3 = 12

• Step 3: Five actions– Market product W– Market product X– Market product Y– Market product Z– Do not market product

An Example• Step 4 & Step 5: :

Conditions 1 2 3 4 5 6 7 8 9 10 11 12

Gender F M F M F M F M F M F M

City Y Y N N Y Y N N Y Y N N

Age group A A A A B B B B C C C C

ActionsMarket W X X XMarket X X XMarket Y XMarket Z X X X X X X X X X XDon’t market

X

Simplify Decision Table• Remove impossible rules• Eliminate indifferent conditions

– An indifferent condition is a condition whose values do not affect the decision and always result in the same action.

– These rules can be consolidated into a single rule

• Simplify unclear rules or actions

An Example1 2 3 4 5 6 7 8

Condition-1 Y Y Y Y N N N NCondition-2 Y Y N N Y Y N NCondition-3 Y N Y N Y N Y N

Action-1 XAction-2 XAction-3 X X XImpossible X X X

An Example• Eliminate impossible rules

1 2 3 4 5Condition-1 Y N N N NCondition-2 Y Y Y N NCondition-3 Y Y N Y N

Action-1 XAction-2 XAction-3 X X X

An Example• Merge rule 4 and 5 to form 4’

1 2 3 4’Condition-1 Y N N NCondition-2 Y Y Y NCondition-3 Y Y N -

Action-1 XAction-2 XAction-3 X X

An Example• Simplify unclear rules

1’ 2 3 4’Condition-1 Y N N NCondition-2 - Y Y NCondition-3 - Y N -

Action-1 XAction-2 XAction-3 X X