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ENGM 620
Fall 2010
Session Seven Homework Solutions
Some of these questions do not have a universal answer. Therefore, the answers are written to
elicit a range of potential answers based on differing perspectives. When this is true, it is
particularly important for the student to concentrate on justifying their selected perspective.
Chapter 11 (12) Discussion Questions
5. What are some applications of process charts in services? Could demerits (points off for
mistakes) be charted? How?
Process charts, as defined in the chapter, are tools for measuring quantifiable data. A
process chart could be used to measure a quantifiable property of a service environment.
This would include items that are time based or measurable such as response time, time
spent to deliver a service or number of complaints.
A demerit could be tracked if a predictable and standardized manner of assigning
the demerits. This technique might contradict the approach to continued improvement.
6. What is random variation? Is it always uncontrollable?
Random variation is that variation in a process that can be measured and
analyzed. If it is controlled, then by definition, it is not random.
A point that needs be made: The word “random” has a very specific
meaning in statistics. Random variables are those values that are all members of
the same set of variables and have an equal probability of occurring. Figure 12-1
demonstrates this property.
7. When would you choose an np chart over a p chart? An X chart over an x-bar chart? An s
chart over an R chart?
Charts are tools for portraying statistical information in an easy-to-comprehend
manner. A p chart presents the proportion of defective parts whereas an np chart
presents the number of non-conforming items. A p chart could be used to
compare different items to observe the difference in the processes.
An X chart is used to evaluate a population. An x¯ (x-bar) chart presents
the same information for a sample. The cost and ease of getting an x¯ chart
obviously make them preferable.
An R chart presents the range of values; this is simply the high-value
minus the low value. An s chart presents the range of standard deviations. The
standard deviation presents the average variance that the sample points are from
the mean or average value. An s chart shows much more detailed information
whereas an R chart shows an overview of the situation
Chapter 11 (12) Problems
5. Interpret the charts given in the book to determine if the processes are stable.
Because of the way the data is presented in the textbook, some of the potential
problems may be hard to see.
a. Five successive points above the centerline: investigate for lack of stability
b. Possible change in level (decrease): investigate for lack of stability
c. Appears to be stable
d. Seven points in a row steadily decreasing: investigate for lack of stability
e. Large jumps (erratic behavior): investigate for lack of stability
f. May be stable, possibly headed toward 15 points in a row within 1 sigma limits,
which would need to be investigated.