CHAPTER 27 • Statistical Process Control

27-44 CHAPTER 27 • Statistical Process Control
CHECK YOUR SKILLS
27.37 A maker of auto air conditioners checks a sample of 4
thermostatic controls from each hour’s production. The thermostats
are set at 75F and then placed in a chamber where
the temperature is raised gradually. The temperature at which
the thermostat turns on the air conditioner is recorded. The
process mean should be 75F. Past experience indicates
that the response temperature of properly adjusted thermostats
varies with 0.5F. The mean response temperature
x for each hour’s sample is plotted on an x control chart. The
center line for this chart is
(a) 0.5F. (b) 75F. (c) 4.
27.38 In Exercise 27.37, the UCL for the chart is
(a) 78.0F. (b) 76.5F. (c) 75.75F.
27.39 In Exercise 27.37 suppose the standard deviation s for
each hour’s sample is plotted on an s control chart. The center
line for this chart is
(a) 0.5F. (b) 0.46F. (c) 0.19F.
27.40 In Exercise 27.37 suppose the standard deviation s for
each hour’s sample is plotted on an s control chart. The LCL
for this chart is
(a) 0F. (b) 0.5F. (c) 1.04F.
27.41 Samples of size 4 are taken from a manufacturing
process every hour. A quality characteristic is measured
and x and s are measured for each sample. The process
is in control and after 25 samples we compute x 48.4
and s 5.1. The LCL for an x control chart based on these
data is
(a) 48.4. (b) 33.1. (c) 31.8.
27.42 A process produces rubber fan belts for automobiles.
The process is in control and 100 belts are inspected each
day for a period of 20 days. The proportion of nonconforming
belts found over this 20-day period is p 0.12. Based on
these data, a p chart for future samples of size 100 would have
center line
(a) 0.12. (b) 12.0. (c) 240.
27.43 Referring to the previous exercise, the UCL for a p
chart for future samples of size 100 would be
(a) 0.22. (b) 0.15. (c) 0.12.
CHAPTER 27
EXERCISES
27.44 Enlighten management. A manager who knows no
statistics asks you, “What does it mean to say that a process
is in control? Is being in control a guarantee that the quality
of the product is good?” Answer these questions in plain
language that the manager can understand.
27.45 Special causes. Is each of the following examples of
a special cause most likely to first result in (i) a sudden change
in level on the s or R chart, (ii)
a sudden change in level on the
x chart, or (iii) a gradual drift
up or down on the x chart? In
each case, briefly explain your
reasoning.
(a) An airline pilots’ union
puts pressure on management
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during labor negotiations by
asking its members to “work to rule” in doing the detailed
checks required before a plane can leave the gate.
(b) Measurements of part dimensions that were formerly
made by hand are now made by a very accurate laser
system. (The process producing the parts does not change—
measurement methods can also affect control charts.)
(c) Inadequate air conditioning on a hot day allows the temperature
to rise during the afternoon in an office that prepares
a company’s invoices.
27.46 Deming speaks. The quality guru W. Edwards
Deming (1900–1993) taught (among much else) that 15
(a) “People work in the system. Management creates the
system.”
(b) “Putting out fires is not improvement. Finding a point
out of control, finding the special cause and removing it,
is only putting the process back to where it was in the first
place. It is not improvement of the process.”
(c) “Eliminate slogans, exhortations and targets for the workforce
asking for zero defects and new levels of productivity.
Choose one of these sayings. Explain carefully what facts
about improving quality the saying attempts to summarize.
27.47 Pareto charts. You manage the customer service
operation for a maker of electronic equipment sold to business