6.12 Mary Beth Marrs, the manager of an apartment
complex, feels overwhelmed by the number of complaints she
is receiving. Below is the check sheet she has kept for the past
12 weeks. Develop a Pareto chart using this information. What
recommendations would you make?
S6.11 Twelve samples, each containing five parts, were taken
from a process that produces steel rods. The length of each rod in
the samples was determined. The results were tabulated and sample
means and ranges were computed. The results were:
SAMPLE SAMPLE MEAN (in.) RANGE (in.)
1 10.002 0.011
2 10.002 0.014
3 9.991 0.007
4 10.006 0.022
5 9.997 0.013
6 9.999 0.012
7 10.001 0.008
8 10.005 0.013
9 9.995 0.004
10 10.001 0.011
11 10.001 0.014
12 10.006 0.009
Total 10.0005 0.0115
b) Draw the charts and plot the values of the sample means and ranges.
c) Do the data indicate a process that is in control?
d) Why or why not? PX
S6.20 Jamison Kovach Supply Company manufactures paper
clips and other office products. Although inexpensive, paper clips
have provided the firm with a high margin of profitability. Sample
size is 200. Results are given for the last 10 samples:
SAMPLE 1 2 3 4 5 6 7 8 9 10
DEFECTIVES 5 7 4 4 6 3 5 6 2 8
a) Establish upper and lower control limits for the control chart and graph the data.
b) Is the process in control?
c) If the sample size were 100 instead, how would your limits and conclusions change? PX
S6.23 The school board is trying to evaluate a new math
program introduced to second-graders in five elementary schools
across the county this year. A sample of the student scores on
standardized math tests in each elementary school yielded the following
SCHOOL NO. OF TEST ERRORS
Center Line 42.6
Construct a c-chart for test errors, and set the control limits
to contain 99.73% of the random variation in test scores.
What does the chart tell you? Has the new math program been
S6.27 Meena Chavan Corp.’s computer chip production process
yields DRAM chips with an average life of 1,800 hours and
σ = 100 hours. The tolerance upper and lower specification limits
are 2,400 hours and 1,600 hours, respectively. Is this process capable
of producing DRAM chips to specification?
S6.35 One of New England Air’s top competitive priorities
is on-time arrivals. Quality VP Clair Bond decided to personally
monitor New England Air’s performance. Each week for the past
30 weeks, Bond checked a random sample of 100 flight arrivals for
on-time performance. The table that follows contains the number of
flights that did not meet New England Air’s definition of “on time”:
a) Using a 95% confidence level, plot the overall percentage of late
flights (p) and the upper and lower control limits on a control
b) Assume that the airline industry’s upper and lower control limits
for flights that are not on time are .1000 and .0400, respectively.
Draw them on your control chart.
c) Plot the percentage of late flights in each sample. Do all samples
fall within New England Air’s control limits? When one falls outside
the control limits, what should be done?
d) What can Clair Bond report about the quality of service? PX