Deepak Sleeba George

 Profile Answers by Deepak Sleeba George 

• Mar 17th, 2008

 

Common-cause variation is characterised by:

• Phenomena constantly active within the system;
• Variation predictable
• Irregular variation within an historical experience base
• Lack of significance in individual high or low values.

Special-cause variation is characterised by:

• New, unanticipated, emergent or previously neglected phenomena within the system;
• Variation inherently unpredictable
• Variation outside the historical experience base; and
• Evidence of some inherent change in the system or our knowledge of it.

Special-cause variation always arrives as a surprise.

kurtz182

 Profile Answers by kurtz182 Questions by kurtz182

• Nov 25th, 2009

 

Common and special causes were introduced by Walter A. Shewhart and Edwards Deming to describe variation in a process through statistical methodologies.  These concepts are used to discern differences in causes when defects are discovered.  Defects that have common causes:  
1) are constantly active within a system,
2) have variations that are predictable,
3) have statistical measurements that typically lack extremely high or low values, and 4) are considered the 'noise' in a system. 

In the software development process, examples of common causes might be defects caused by:
1) the nature of an organization's software development and test processes, or
2) the current level of an organization's collective development and test expertise.

Special causes:
1) are new phenomena that had not been expected,
2) have variations that are unpredictable,
3) are evidence of a change in the system,
4) are considered issues that become known when they rise above the 'noise' in the system. 

In the software development process, examples of special causes might be defects caused by:
1) inaccurate or unclear requirements, or
2) an individual's poor software development or test performance due to some unexpected reason (ex: personal problems impacting job performance).