1.Histogram or stem-and-leaf plot

2.Check sheet

3.Pareto chart

4.Cause-and-effect diagram

5.Defect concentration diagram

6.Scatter diagram

7.Control chart

Chance Causes of Quality Variation.

• The natural variability present even in well maintained processes is called "stable system of chance causes".  
• This natural variability or “background noise” is the cumulative effect of many small, essentially unavoidable causes. 
• A process that is operating with only

chance causes of variation present is said to be in statistical control.

Other kinds of variability usually arises from three sources:

• improperly adjusted or controlled machines,
• operator errors,
• or defective raw material.

Such variability is generally large when compared to the background noise, and it usually represents an unacceptable level of process performance. They are not part of the chance cause pattern. A process that is operating in the presence of assignable causes is said to be an out-of-control process.

1. Quickly detect the occurrence of assignable causes of process and take corrective action before many nonconforming units are manufactured.
2. Elimination of variability in the process.

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w  - a sample statistic that measures some quality characteristic

mw  -  mean of w

σw   -  Standard Deviation of w

L   - “distance” of the control limits from the center line, expressed in standard deviation units.