• Significant variability exists in the outputs of different individuals, for instance:
• Design defects found by different reviewers
• Defect “density” for designs done by different individuals
• Bugs found by different code reviewers
• Code defect density for different developers
• Defect turn around time for different individuals
• Different testers productivity (tests executed or bugs found)
• Different testers accuracy (“good” problems found)
• Significant invariance exists in the output of a single individual, for instance:
• Design defects found by same reviewer
• Defect “density” for designs done by same individual
• Bugs found by same code reviewer
• Code defect density for same developer
• Defect turn around time for same individual
• Same tester productivity (tests executed or bugs found)
• Same tester accuracy (“good” problems found)

Why is that so and what is the consequence?