It is often said that “you should control based on facts” ， “you should make decision based on facts” or “you should speak based on data” in quality management.
Even if the products are made under controlled process, the quality of product could vary if process factors such as 4M-E （man，machine，material，method，environment）fluctuate. It is important to understand the fluctuation of factors objectively by data not subjectively， and then you should make decision upon it. Such approach is called the scientific approach since it is based on facts.
Seven QC tools are often used to summarize data:( graph， histogram，fishbone chart，check sheet,scatter diagram, Pareto chart,stratification)
After analyzing data you should take action to the group from where sample data is taken，not to the sample itself.
Standard deviation, Guide to rounding numbers, Priority approach, Design of experiments，Sampling inspection