References 
1. TTT, "Quality Systems for Suppliers", Volume 5, Page 32-34. 
2. NNN, "Quality Assurance for Finished Products", Edition 3, Page 21-24. 
3. PPP, "Product Quality and Safety, Edition 5, Page 26-28. 
4. DDD, "Quality Control and Quality Assurance", Volume 4, Page 19-22. 
5. http://www.tso.co.uk/british_standards, March 2004. 
6. James Charington, "Quality at Needle Point", Edition 2, Page 33. 
7. David J Taylor, "Materials Management", Edition 2, Page 14.