TABLE OF CONTENTS Quality defects can be calculated from the defect rate generated by six sigma or Cpk, from the interaction of the production process and the specification limits. The production process characteristics are assumed to be normally distributed. This distribution is also known as the bell curve, and is symmetrical. The area under the curve is equal to 1, and it is much smaller on both ends, as shown in Figure 2.8. Once a process is determined to be normally distributed, it can be characterized by two numbers: a process average ? and a population standard deviation ?. A standard normal curve is one that has an average ? = 0 and ? = 1. For each value z in the x-axis, the area under the curve is given as f( z) in Table 2.3. This area is determined from x = ? ? to x = z. Sometimes this normal distribution is called the z distribution, where z is the normalized value of the x-axis intercept.
| z | f( z) | z | f( z) | z | f( z) | z | f( z) | z | f( z) | z | f( z) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0.5 | ||||||||||
| ?0.01 | 0.50399 | 1.01 | 0.84375 | 2.01 | 0.97778 | 3.01 | 0.99869 | 4.01 | 0.99996963 | 5.01 |
