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The normal distribution (also known as the Gaussian distribution) is a continuous probability distribution. Most processes that are 'in control' conform to a normal distribution, or to some variant of it. Normality is also a condition of many statistical tests.
The normal distribution has a 'bell shape':
The proportion of the process output that falls within a range of values is represented by the area of the curve. Values are:
| Between -1 and +1 Standard Deviation |
68.3% (about two thirds) |
| Between -2 and +2 Standard Deviation |
95.5% (about 95%) |
| Between -3 and +3 Standard Deviation |
99.7% |
All values can be obtained from the Normal Distribution Tables.
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