CHP 8

Which of the following charts is used to monitor the average number of nonconformances per unit when sample sizes vary?

u-chart

Process capability indices can be computed from data in an x-chart.

True

Rolled throughput yield is the proportion of conforming units that results from a series of process steps.

True

The center line for the np-chart is the average number of nonconforming items per sample.

True

The larger the sample size, the narrower the control limits.

True

Twenty samples of size 6 were collected for a variable measurement. Determine the upper control limit (UCL) and lower control limit (LCL) for an R-chart if the sum of the sample ranges equals 88.0.

UCL = 8.8176, LCL = 0.0000

All else being constant, if the sample size in a p-chart increases from 100 to 120, the upper and lower control limits will:

Move Closer together

The control chart most appropriate for plotting the daily percentage of students that wait longer than 5 minutes in line at the undergraduate admissions office is a(n):

P-chart

Attribute measurement requires a much larger sample size than variable measurement to obtain the same amount of statistical information.

True

Control limits for variables and attributes data utilize different mathematical formulas.

True

Pre-control should only be used when process capability is no greater than 88 percent of the tolerance

true

The frequency of sampling in pre-control is often determined by dividing the time period between two successive put-of-control signals by six

true

A c-chart is based on the binomial distribution.

False

25 samples of size 100 were collected and a total of 96 nonconforming items were found. The upper control limit for a p-chart would be:

0.096.

Which of the following is a likely cause of the control chart pattern known as a "trend"?

Aging equipment

The _____ is used to monitor the centering of the process

A. X-chart

For variable data, the two important quality measures are the proportion nonconforming and nonconformances per unit

false

common and special causes of variation must be eliminated before process capability can be determined

false

histograms allow you to distinguish between common and special causes of variations

false

To form rational subgroups for an -chart, the goal is to choose the samples as _____ as possible.

homogeneous

. A canning company defines a "nonconformance" as an orange juice can whose diameter is beyond the upper specification limit. A manager at the canning company wants to monitor the number of such orange juice cans produced per day. Each day, an employee will randomly select 120 cans over a period of four weeks. The employee then determines and plots the number of nonconformance cans in each sample. The appropriate control chart is a(n):

np-chart

A _____ monitors the fraction of nonconforming units.

p-chart

A delivery company defines a "defect" as a package delivered later than the promised delivery time. Management wants to monitor the proportion of such defect packages per week and plans to sample 120 packages every week. The appropriate control chart is a(n):

p-chart

A p-chart is used with the attribute data.

true

If most of the points on a control chart fluctuate randomly between the control limits with no recognizable pattern, then the process is in statistical control.

true

If points on a control chart are determined to be the result of special causes, the points should be eliminated and new control limits should be computed.

true

A measurement system may be precise but not necessarily accurate at the same time.

True

The sample size for an x-chart:

is equal to one.

A precision parts manufacturer produces bolts for use in military aircraft. The specifications for bolt length are 37.50 ± 0.25 cm. The company has established an -chart and an R-chart using samples of size five. The center lines for the -chart and R-chart are set at 37.35 cm and 1.05 cm, respectively. What is the value of Cpk for this process?

A. 0.07

Which of the following is NOT an indication of an out-of-control process? a. Five consecutive points fall on one side of the center line. b. Four of five consecutive points fall within the outer two-thirds region between the center line and one of the control limits. c. Two of three consecutive points fall in the outer one-third region between the center line and one of the control limits. d. A single point falls above the upper control limit.

A. Five consecutive points fall on one side of the center line.

In pre-control, which zone covers the nominal specification of a process? a. Gray zone b. Green zone c. Red zone d. Yellow zone

B green zone

Defects per million opportunities (dpmo) = _____.

B. (Number of defects discovered / Opportunities for error) x 1,000,000

A precision parts manufacturer produces bolts for use in military aircraft. The specifications for bolt length are 37.50 ± 0.25 cm. The company has established an -chart and an R-chart using samples of size five. The center lines for the -chart and R-chart are set at 35.25 cm and 1.08 cm, respectively. What is the value of Cp for this process?

B. 0.179

_____ is measured as the amount of error in a measurement in proportion to the total size of the measurement. a. Precision b. Accuracy c. Systematic bias d. Capability

B. Accuracy

For variables data, the sample means are assumed to be:

B. Normally Distributed

A company wishes to monitor the variation in the length of time it takes to process and send a bill to a customer. Seven bills will be randomly selected each day for a 60-day period and the length of time it takes to process and send the bill to a customer will be recorded for each bill. The appropriate control chart is a(n):

B. R-chart

A process is said to be out of control when: a. common causes are present. b. special causes are present. c. the process is not capable of meeting specifications. d. the upper and lower control limits cannot be computed.

B. Special causes are present

Which of the following is NOT one of the basic applications of a control chart?

B. To calculate the average-range.

Short repeated patterns in a control chart, with alternating high peaks and low valleys, are known as:

B. cycles

The output of a process is stable and normally distributed. If the process mean equals 23.5, the percentage of output expected to be less than or equal to the mean: a. is less than 25%. b. is 50%. c. is greater than 75%. d. cannot be determined without knowing the standard deviation value.

B. is 50%

Process capability calculations make little sense if the process: a. data were taken over a period of time. b. is not in statistical control. c. variations are due to common causes. d. is not initiated.

B. is not in statistical control.

If a process is neither capable nor in control, the appropriate first step is to: a. remove common causes of variation. b. redesign the equipment. c. remove special causes of variation. d. determine the process capability index.

C . remove special causes of variation.

_____ is a study of variation in a measurement system using statistical analysis. a. Metrology study b. SPC study c. A repeatability and reproducibility (R&R) study d. Calibration study

C. A repeatability and reproducibility (R&R) study

A(n) _____ characterizes the presence or absence of nonconformances in a unit of work

C. Attribute

Collecting _____ data is usually easier than collecting _____ data because the assessment can usually be done more quickly by a simple inspection or count.

C. Attribute; Variable

Statistical process control relies on _____.

C. Control Charts

If the process mean and variance do not change over time, the process is considered to be:

C. Stable

Compared to variable measurement, attributes measurement requires _____ to obtain the same amount of statistical information about the quality of the product.

C. a large sample

If the variation in the process is due to common causes alone, the process is said to be: a. out of control. b. in pre-control. c. in statistical control. d. out of capability.

C. in statistical control

x-charts are used for:

C. individual variables data.

A _____ is the result of some cause that gradually affects the quality characteristics of the product and causes the points on a control chart to gradually move up or down from the center line.

D. Trend

Which of the following pairs of charts are used together?

D. x-chart and R-chart

A "hugging the center line" pattern in a control chart can be the result of taking each sample from a different process but not mixed together.

False

A better alternative to the -chart to monitor variation is to compute and plot the standard deviation s of each sample

False

A nonconforming unit of work is one that has no defects or errors.

False

Control limits for the s-chart are given by: LCLs = B3s and UCLs = B4s, where B3 and B4 are constants.

False

If all sample averages on an -chart fall within the control limits, all output will be conforming.

False

In determining the method of sampling, samples should be chosen to be as heterogeneous as possible so that each sample reflects the system of common causes or assignable causes that may be present at that point in time.

False

If the cost of investigating an operation to identify the cause of an apparent out-of-control condition is low, narrower control limits should be selected

True

The total observed variation in production output is the sum of the true process variation plus variation due to measurement

True

When constructing a u-chart, the standard deviation of each sample varies because the size of the sampling unit varies.

True

If the lower limit for a p-chart computes to a number less than zero, this indicates

a. that the lower limit should be set at zero.

A machined part has a lower specification limit of 3.5 inches and an upper specification limit of 3.9 inches. The range for the pre-control green zone is:

a. 3.6 inches to 3.8 inches.

In determining whether a process is in statistical control, the _____ is/are generally analyzed first

a. R-chart

The s-chart is recommended when:

b. tight control of variability in the process is required

Which of the following is CORRECT regarding an x-chart?

b. It is less sensitive to many conditions that can be detected by -chart and R-chart.

An alternative to using the R-chart is the:

b. s-chart.

Six samples of size 10 were collected. The data are provided in the following table: Sample number 123 4 5 6 Sample mean 4.80 4.62 4.81 4.55 4.92 4.65 Sample standard deviation 0.35 0.33 0.31 0.35 0.37 0.39

b.LCL = 0.0994, UCL = 0.6006.

Each day, a manufacturing company producing circuit boards will randomly sample 60 circuit boards for a period of 25 days and then determine and plot the number of defects contained in each of these boards. The appropriate control chart is a(n):

c-chart.

c-charts are based on the _____ probability distribution.

c. Poisson

Which of the following is a major advantage of np-charts?

c. They are easier to understand for production personnel.

Because of high test costs, a sample of single wastewater sludge is tested daily for dissolved oxygen content. The appropriate control chart is:

c. x-chart and moving range chart.

Variable Data Are

c. continuous

In service applications, the term _____ is generally used to describe a nonconformance.

c. error

Control limits are often confused with _____.

c. specification limits

76. For statistical process control, a good sampling method should always:

d. use rational subgroups.

A histogram of data that resembles the normal distribution indicates that the process is in control.

false