If we want to find all inconsistencies in a requirements document, what will we need to do?
Inconsistency is more than two conflicting logical or other simple states. A person can’t be in two places at once (but they can communicate to two places at once, unless communication is directional).
Some seem easy:
The system cabinet shall be coloured red.
The system cabinet shall be coloured blue.
The system shall consist of a computer and modem.
The system shall not use a modem.
"use" is not the same as "include". We could have
The system shall not include a modem.
The system shall use the general office modem.
The use could be qualified:
The system shall consist of a computer and modem.
The system shall not use a modem to communicate with the printer.
This is already ambiguous – the system may use a LAN to establish connection, the LAN communicating with the printer through a modem. If it said "the", implying the modem in the system, the ambiguity would be removed.
During Inconsistency Checking, we will need to loosen the distinctions among some relations, such as ToInclude, ToConsistOf, ToComprise, ToInstall, ToUse, while ensuring we pick up on qualification that removes inconsistency.
If we have a statement indicating what cannot be used, and we have no statement saying what can be used, do we need to set up an alternative statement, such as
The system shall communicate with the printer, without using a modem.
The system shall be powered by 240 VAC.
If the system is powered by alternating current, it shall include an Earth Leakage Circuit
Breaker.
The conditional is unnecessary, but does that make it inconsistent?
Direct Contradiction
The application software shall be installed on each SAX computer according to
installation procedures.
A SAX computer shall not include any software in addition to the Operating System.
Need to know that "installed on" equates to "include", but otherwise the contradiction is direct.
Indirect Contradiction
The system shall support one person operation.
There are two operator consoles, at two different locations.
Operations are required at each console, potentially simultaneously. One person operation is not possible.
Inconsistency from Prior Knowledge
The construct of a Tow Motor shall ensure the ingress of air-borne dust and sand into the cabin.
We have a model of a comfortable working environment - 15 - 25º C, 20-70 dBA, a maximum particulate level – increasing the particulate content by allowing ingress is inconsistent with that, but having a cabin heater is not.
The turning radius of the Tow Motor shall not exceed 15 m.
Truck typically 12-20m turning diameter. Inheritance and numeric conversion.