L. P. Provost and C. L. Norman, Variation through the Ages
(Quality Progress, December 1990)
Reading guide
Some parts of this paper will be discussed in more detail in upcoming lectures. In
particular, the issues raised on pp. 42-43 will be explained thoroughly during the
lectures. However, in order to better understand the lectures, it is recommended to
study these pages briefly and answer the questions presented below.
Questions
1.
2.
3.
4.
5.
Describe the three methods of managing variation mentioned in the text.
Describe the different test methods mentioned in this text.
Why was variation not a critical issue in the period before 1700?
When and why did variation become a critical issue?
What was the reason that standards for inspection were formulated at the end of the
19th century?
6. Explain the go/ no-go principle.
7. What is the goal of setting tolerances and specifications?
8. Explain the term interchangeability and give an example of a 'modern' product with
interchangeable components.
9. Explain the relation between interchangeable components and variation.
10. What is the goal of a Shewhart control chart?
11. In what sense does Shewhart’s approach differ from the approach based on setting
specifications?
12. What were the reasons for Shewhart to put control limits at a distance of 3
standard deviations of the target value?
13. Look up the section on Chebyshev’s inequality in your probability and statistics text
book. What are the probabilities of being outside one of the control limits if the
control limits are at 2  or 3.5 , respectively? For which type of distribution can
we use Chebyshev’s inequality to obtain probabilities for being outside the upper
control limit?
14. In what sense was the Shewhart approach based on control charts better for workers
than the Taylor approach based on specifications and inspection.
15. Explain the idea behind Taguchi's quadratic loss function.
16. Explain what is meant by tolerance stack-up (see p. 44, 1st paragraph below
figure 3).
17. If 2 components each have a deviation of target weight which is normally distributed
with mean 0 and variance 2, what is the probability that the deviation of the total
weight of the 2 components falls outside control limits set at 3?