Deck 11: Statistically Based Quality Improvement for Variables

Question 1

Discuss the concept of control. Is control helpful? Isn't being controlling a negative?

Answer 1

Control is also an important managerial function like plan, organize, recruitment and directing.
Yes, this control function is used to check errors in the samples and helps to take corrective actions. Also the control function is used to reduce the deviation from standards, and avowed targets of the organization are attained in a desired manner.
In earlier methods, control was used to find out the errors in the samples. But in this modern world, control is used to setting standards, measuring real performance and taking corrective action.
Now control function is considered as the following activities:
• A continuous process,
• Managing process,
• Advance looking process,
• Closely associated with planning,
• Embedded with every stage of managerial hierarchy,
• Consider as a tool to achieve organizational activities,
• An end process.
Hence, the concept of control function is discussed.

Question 2

Return to the chart in Figure. Is this process stable? Explain.
FIGURE Completed x and R Chart

Answer 2

Refer to FIGURE 11.8 in the textbook. The process is not stable due to the following reasons:
• In this chart, subgroup 3 and 8 violated the control limits.
• Sample 3 is above the upper control limit and sample 8 is below the lower control limit.
• There are no patterns or drifts on the chart.
• Finally, entire points are not balanced between the limits.
Hence, it can be concluded that this chart does not contain a stable control. Therefore, assignable causes for the variation in process should be investigated.

Question 3

The concept of statistical thinking is an important theme in this chapter. What are some examples of statistical thinking?

Answer 3

The concept of statistical thinking is a decision-making skill established by the capability to illustrate conclusion based on information.
The statistical thinking is attentiveness on the deviation, and is the fiscal point.
The statistical thinking is used to allow the permissible limits in upper and lower side without affecting the quality of the product.
Examining the quantity of sample products, it allows sustaining a continuing quality control.
Measurement of sugar level in human blood before and after taking breakfast is varied in limited time period. Based on the measurement data, statistical thinking concludes the corrective actions.
Hence, the examples of statistical thinking concept are discussed.

Question 4

Return to the data in Figure. Is this process capable? Compute both Cpk and Ppk.
FIGURE Completed x and R Chart

Question 5

Sometimes you do well on exams. Sometimes you have bad days. What are the assignable causes when you do poorly?

Question 6

For the following product characteristics, choose where to inspect first:

Question 7

What is the relationship between statistical quality improvement and Deming's 14 points?

Question 8

For the following product characteristics, choose where to inspect first:

Question 9

What are some applications of process charts in services? Could demerits (points off for mistakes) be charted? How?

Question 10

Interpret the following charts to determine if the processes are stable.

Question 11

What is random variation? Is it always uncontrollable?

Question 12

Interpret the following charts to determine if the processes are stable.

Question 13

When would you choose an np chart over a p chart? An X chart over an
chart? An s chart over an R chart?

Question 14

Tolerances for a new assembly call for weights between 32 and 33 pounds. The assembly is made using a process that has a mean of 32.6 pounds with a population standard deviation of.22 pounds. The process population is normally distributed.
a. Is the process capable?
b. If not, what proportion will meet tolerances?
c. Within what values will 99.5% of sample means of this process fall if the sample size is constant at 10 and the process is stable?

Question 15

Design a control chart to monitor the gas mileage in your car. Collect the data over time. What did you find?

Question 16

Specifications for a part are 62? +/?.01. ? The part is constructed from a process with a mean of 62.01? and a population standard deviation of.033. The process is normally distributed.
a. Is the process capable?
b. What proportion will meet specifications?
c. Within what values will 95% of sample means of the process fall if the sample size is constant at 5 and the process is stable?

Question 17

What does "out-of-control" mean? Is it the same as a "bad hair day"?

Question 18

Tolerances for a bicycle derailleur are 6 cm +/?.001 cm. The current process produces derailleurs with a mean of 6.0001 with a population standard deviation of.0004. The process population is normally distributed.
a. Is the process capable?
b. If not, what proportion will meet specs?
c. Within what values will 75% of sample means of this process fall if the sample size is 6 and the process is stable?

Question 19

Design a control chart to monitor the amounts of the most recently charged 50 debits from your debit card. What did you find?

Question 20

A services process is monitored using
and R charts. Eight samples of n = 10 observations have been gathered with the following results:
a. Using the data in the table, compute the center line, the upper control limit, and the lower control limit for the
and R charts.
b. Is the process in control? Interpret the charts.
c. If the next sample results in the following values (2.5, 5.5, 4.6, 3.2, 4.6, 3.2, 4.0, 4.0, 3.6, 4.2), will the process be in control?

Question 21

A production process for the JMF Semicon is monitored using
and R charts. Ten samples of n = 15 observations have been gathered with the following results:
a. Develop a control chart and plot the means.
b. Is the process in control? Explain.

Question 22

Experiment : Randomly select the heights of at least 15 of the students in your class.
a. Develop a control chart and plot the heights on the chart.
b. Which chart should you use?
c. Is this process in control?

Question 23

A finishing process packages assemblies into boxes. You have noticed variability in the boxes and desire to improve the process to fix the problem because some products fit too tightly into the boxes and others fit too loosely. Following are width measurements for the boxes.
Using
and R charts, plot and interpret the process.

Question 24

For the data in Problem, if the mean specification is 68.5 ±.25 and the estimated process standard deviation is.10, is the process capable? Compute Cpu, Cpl, and Cpk.
Problem
A finishing process packages assemblies into boxes. You have noticed variability in the boxes and desire to improve the process to fix the problem because some products fit too tightly into the boxes and others fit too loosely. Following are width measurements for the boxes.
Using
and R charts, plot and interpret the process.

Question 25

For the data in Problem 1, treat the data as if they were population data, and find the limits for an X chart. Is the process in control? Compare your answer with the answers to Problem 2. Hint : Use the formula CL x =
± (3/ d 2 )
(Figure).
Problem 1
A finishing process packages assemblies into boxes. You have noticed variability in the boxes and desire to improve the process to fix the problem because some products fit too tightly into the boxes and others fit too loosely. Following are width measurements for the boxes.
Using
and R charts, plot and interpret the process.
Problem 2
For the data in Problem 1, if the mean specification is 68.5 ±.25 and the estimated process standard deviation is.10, is the process capable? Compute Cpu, Cpl, and Cpk.
Figure

Question 26

A Rochester, New York, firm produces grommets that have to fit into a slot in an assembly. Following are dimensions of grommets (in millimeters):
a. Use
and R charts to determine if the process is in control.

Question 27

Using the data from Problem, compute the limits for
and s charts. Is the process still in control?
Problem
A finishing process packages assemblies into boxes. You have noticed variability in the boxes and desire to improve the process to fix the problem because some products fit too tightly into the boxes and others fit too loosely. Following are width measurements for the boxes.
Using
and R charts, plot and interpret the process.

Question 28

Using the data from Problem, compute the limits for
and s charts. Is the process still in control?
Problem
A Rochester, New York, firm produces grommets that have to fit into a slot in an assembly. Following are dimensions of grommets (in millimeters):
a. Use
and R charts to determine if the process is in control.

Question 29

Use a median chart to determine if the process for the following data is centered.

Question 30

Use an
chart to determine if the data in Problem are in control. Do you get the same answer?
Problem
Use a median chart to determine if the process for the following data is centered.

Question 31

The following data are for a component used in the space shuttle. Because the process dispersion is
closely monitored, use an
and s chart to see if the process is in control.

Question 32

Develop an R chart for the data in Problem. Do you get the same answer?
Problem
The following data are for a component used in the space shuttle. Because the process dispersion is closely monitored, use an
and s chart to see if the process is in control.

Question 33

Using the data from Problem, compute limits for a median chart. Is the process in control?
Problem
The following data are for a component used in the space shuttle. Because the process dispersion is closely monitored, use an
and s chart to see if the process is in control.

Question 34

Design a control plan for exam scores for your quality management class. Describe how you would gather data, what type of chart is needed, how to gather data, how to interpret the data, how to identify causes, and remedial action to be taken when out-of-control situations occur.

Question 35

For the sampling plan from Problem, how would you measure process capability?
Problem
Design a control plan for exam scores for your quality management class. Describe how you would gather data, what type of chart is needed, how to gather data, how to interpret the data, how to identify causes, and remedial action to be taken when out-of-control situations occur.

Question 36

For the data in Problem, if the process target is 50.25 with spec limits +/?5, describe statistically the
problems that would occur if you used your spec limits on a control chart where n = 5. Discuss type I
and type II error.
Problem
A Rochester, New York, firm produces grommets that have to fit into a slot in an assembly. Following are dimensions of grommets (in millimeters):
a. Use
and R charts to determine if the process is in control.