BS7925-2 Annex B - Guidelines for Testing and Test Measurement

Guidelines for Testing Techniques and Test Measurement

B.4    Cause Effect Graphing

Introduction

This black box technique is based upon an analysis of the specification of the component to model its behaviour by means of causes and effects.  We illustrate the technique by means of a worked example.  Naturally, the technique is only effective to the extent that the model captures the specification of the component.

Example

Take a cheque debit function whose inputs are debit amount, account type and current balance  and whose outputs are new balance and action code.   Account type may be postal ('p') or counter ('c').  The action code may be 'D&L', 'D', 'S&L' or 'L', corresponding to 'process debit and send out letter', 'process debit only', 'suspend account and send out letter' and 'send out letter only' respectively.  The function has the following specification:

If there are sufficient funds available in the account or the new balance would be within the authorised overdraft limit then the debit is processed.  If the new balance would exceed the authorised overdraft limit then the debit is not processed and if it is a postal account it is suspended.  Letters are sent out for all transactions on postal accounts and for non-postal accounts if there are insufficient funds available (i.e. the account would no longer be in credit).

 

The conditions are:

C1           New balance in credit

C2           New balance overdraft, but within authorised limit

C3           Account is postal

 

The actions are:

A1           Process debit

A2           Suspend account

A3           Send out letter

 

A cause-effect graph shows the relationship between the conditions and actions in a notation similar to that used by designers of hardware logic circuits.  The specification is modelled by the graph shown in Figure B.4.

         

Figure B.4: Cause-effect graph

The code graph is then recast in terms of a decision table.  Each column of the decision table is a rule.  The table comprises two parts.  In the first part each rule is tabulated against the conditions.  A 'T' indicates that the condition must be TRUE for the rule to apply and an 'F' indicates that the condition must be FALSE for the rule to apply.  In the second part, each rule is tabulated against the actions.  A 'T' indicates that the action will be performed; an 'F' indicates that the action will not be performed; an asterisk (*) indicates that the combination of conditions is infeasible and so no actions are defined for the rule.

The example has the following decision table:

Rules:	1	2	3	4	5	6	7	8
C1: New balance in credit	F	F	F	F	T	T	T	T
C2: New balance overdraft, but within authorised limit	F	F	T	T	F	F	T	T
C3: Account is postal	F	T	F	T	F	T	F	T
A1: Process debit	F	F	T	T	T	T	*	*
A2: Suspend account	F	T	F	F	F	F	*	*
A3: Send out letter	T	T	T	T	F	T	*	*

The following test cases would be required to provide 100% cause-effect coverage, and correspond to the rules in the decision table above (no test cases are generated for rules 7 and 8 as they are infeasible):