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Unit 8: Statistical Quality Control - Practice Quiz

ECAP790 60 Questions
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1 What is the main purpose of statistical quality control?

Introduction to statistical quality control Easy
A. To calculate employee salaries
B. To monitor and improve quality
C. To design product advertisements
D. To determine selling prices

2 Which type of variation is naturally present in a stable production process?

Introduction to statistical quality control Easy
A. Inspection variation
B. Assignable-cause variation
C. Common-cause variation
D. Variation caused by a specific machine failure that requires immediate corrective maintenance

3 A process is said to be in statistical control when it is affected only by:

Process control Easy
A. Defective materials
B. Special causes
C. Common causes
D. Measurement errors

4 What does a point outside the control limits usually indicate?

Process control Easy
A. A guaranteed acceptable product
B. A change in sample size
C. A possible special cause
D. A perfectly stable process

5 What does an chart monitor?

Control charts for variables: X and R charts, X and S charts Easy
A. The process mean
B. The defect count
C. The defective proportion
D. The process range

6 What does an chart monitor?

Control charts for variables: X and R charts, X and S charts Easy
A. Number of defects
B. Process location
C. Defective proportion
D. Process variability

7 How is the sample range calculated?

Control charts for variables: X and R charts, X and S charts Easy
A. Maximum plus minimum
B. The sum of all observations divided by the total number of observations
C. Maximum minus minimum
D. Mean minus median

8 Which pair of charts is commonly used to monitor a process mean and range?

Control charts for variables: X and R charts, X and S charts Easy
A. and charts
B. and charts
C. and charts
D. and charts

9 What does an chart use to measure process variability?

Control charts for variables: X and R charts, X and S charts Easy
A. Sample proportion
B. Sample defect count
C. Sample standard deviation
D. Sample median

10 Which chart is paired with an chart to monitor the process average?

Control charts for variables: X and R charts, X and S charts Easy
A. chart
B. chart
C. chart
D. chart

11 When are and charts generally preferred over and charts?

Control charts for variables: X and R charts, X and S charts Easy
A. For attribute observations
B. When every produced item is inspected and all defective units are immediately discarded
C. For larger sample sizes
D. For counting defectives

12 What quantity is plotted on a chart?

Control charts for attributes: p chart Easy
A. Sample range
B. Proportion defective
C. Number of defects
D. Sample average

13 Which type of data is used with a chart?

Control charts for attributes: p chart Easy
A. Defective or nondefective data
B. Time-series average data
C. Measurements of length, mass, temperature, and pressure recorded for every individual item
D. Continuous measurement data

14 A sample contains 10 defective units out of 100 inspected units. What is the sample proportion defective?

Control charts for attributes: p chart Easy
A.
B.
C.
D.

15 What quantity is plotted on an chart?

Control charts for attributes: np chart Easy
A. Proportion of defective units
B. Number of defects per unit
C. Average measured value
D. Number of defective units

16 An chart is most appropriate when sample sizes are:

Control charts for attributes: np chart Easy
A. Unknown
B. Different for every sample because production volume changes throughout each working day
C. Constant
D. Continuously changing

17 What does a chart monitor?

Control charts for attributes: c chart Easy
A. Number of defectives
B. Proportion defective
C. Number of defects
D. Process average

18 Which situation is suitable for a chart?

Control charts for attributes: c chart Easy
A. Finding the average bottle weight
B. Counting scratches on each sheet
C. Classifying each manufactured component as either acceptable or defective after a complete inspection
D. Measuring the length of each rod

19 What is a primary application of a control chart?

Applications of control charts in process control Easy
A. Setting product prices
B. Detecting process changes
C. Selecting advertising channels
D. Preparing sales reports

20 If all plotted points are randomly distributed within the control limits, the process is generally considered:

Applications of control charts in process control Easy
A. In statistical control
B. Completely defect-free
C. Affected by a major special cause that must be removed before production can continue
D. Outside specification

21 A machine produces components whose diameters vary slightly because of temperature, vibration, and occasional tool damage. Which variation should statistical quality control primarily identify for corrective action?

Introduction to statistical quality control Medium
A. Systematic variation caused by identifiable tool damage
B. Small random variation caused by many unavoidable factors
C. Expected variation caused by measurement rounding
D. Natural variation occurring within established control limits

22 Which statement best distinguishes a control chart from acceptance sampling?

Introduction to statistical quality control Medium
A. A control chart evaluates submitted lots, while acceptance sampling estimates process capability
B. A control chart measures customer satisfaction, while acceptance sampling measures productivity
C. A control chart determines specifications, while acceptance sampling establishes control limits
D. A control chart monitors a process over time, while acceptance sampling evaluates submitted lots

23 A process has all plotted points within its control limits, with no runs, trends, or unusual patterns. What is the most appropriate conclusion?

Process control Medium
A. The process produces no defective items
B. The process necessarily meets all product specifications
C. The process is operating with only common-cause variation
D. The process mean is exactly equal to the target value

24 A process is statistically stable, but many products fall outside the engineering specifications. Which conclusion is most appropriate?

Process control Medium
A. The process is neither measurable nor stable
B. The process is both capable and in control
C. The process is capable but not in control
D. The process is in control but not capable

25 For an chart based on samples of size , the grand mean is and . Given , what are the three-sigma control limits?

Control charts for variables: X and R charts, X and S charts Medium
A. and
B. and
C. and
D. and

26 For an chart with samples of size , , , and . A new sample has . What does the chart indicate?

Control charts for variables: X and R charts, X and S charts Medium
A. The range is out of control because it is below the lower limit
B. The range is in control because it is near the center line
C. The range is out of control because it exceeds the upper limit
D. The range cannot be evaluated without the sample mean

27 When subgroup sizes are moderately large, why is an and chart often preferred to an and chart?

Control charts for variables: X and R charts, X and S charts Medium
A. The range measures location rather than dispersion in large samples
B. The standard deviation eliminates the need to calculate subgroup means
C. The range becomes impossible to calculate for moderately large samples
D. The standard deviation uses variability information more efficiently than the range

28 An chart paired with an chart has , , and . What are the limits for the chart?

Control charts for variables: X and R charts, X and S charts Medium
A. and
B. and
C. and
D. and

29 For an chart, , , and . A sample standard deviation of is observed. What is the appropriate interpretation?

Control charts for variables: X and R charts, X and S charts Medium
A. Variability is stable because is below the center line
B. Variability has increased because exceeds the upper limit
C. Variability cannot be assessed until the sample mean is plotted
D. Variability has decreased unusually because is below the lower limit

30 Three samples contain defectives out of , out of , and out of . What is the pooled fraction defective ?

Control charts for attributes: p chart Medium
A.
B.
C.
D.

31 A chart has and a constant sample size of . What are its approximate three-sigma control limits?

Control charts for attributes: p chart Medium
A. and
B. and
C. and
D. and

32 A factory uses a chart with unequal sample sizes. How should its control limits generally be calculated?

Control charts for attributes: p chart Medium
A. Use separate limits based on each sample size
B. Use identical limits based on the largest sample size
C. Use identical limits based on the average defect count
D. Use separate limits based only on each defect count

33 Which production situation is most suitable for an chart?

Control charts for attributes: np chart Medium
A. Monitoring the number of defects on areas of varying size
B. Monitoring the average weight of constant-sized samples
C. Monitoring the number defective in samples of constant size
D. Monitoring the fraction defective when sample sizes vary

34 An chart uses samples of items and has . What are the approximate three-sigma control limits?

Control charts for attributes: np chart Medium
A. and
B. and
C. and
D. and

35 For a constant sample size , how are the plotted values on an chart related to those on a chart?

Control charts for attributes: np chart Medium
A. Each value equals the corresponding value divided by
B. Each value equals the square root of the corresponding value
C. Each value equals one minus the corresponding value
D. Each value equals the corresponding value multiplied by

36 Which condition is required for appropriately using a chart?

Control charts for attributes: c chart Medium
A. The inspection opportunity must remain approximately constant
B. The measured characteristic must follow a normal distribution
C. The number of inspected units must increase over time
D. The sample must contain only one possible defect type

37 The average number of surface defects per sheet is . What are the three-sigma limits for a chart?

Control charts for attributes: c chart Medium
A. and
B. and
C. and
D. and

38 A company counts flaws on fabric pieces whose inspected areas differ substantially. Why is a standard chart inappropriate?

Control charts for attributes: c chart Medium
A. A chart requires every inspected item to be defective
B. A chart requires measurements on a continuous scale
C. A chart requires the average defect count to equal zero
D. A chart requires approximately equal opportunities for defects

39 An chart shows one point above its upper control limit, while the corresponding chart remains in control. What should the process engineer investigate first?

Applications of control charts in process control Medium
A. A possible shift in the process mean
B. A possible increase in within-sample variation
C. A possible error in the specification limits
D. A possible reduction in the subgroup size

40 Seven consecutive points on a control chart increase steadily, although none exceeds the control limits. What is the best interpretation?

Applications of control charts in process control Medium
A. The pattern suggests a nonrandom trend that should be investigated
B. The control limits should be widened to include future observations
C. The process mean must be exactly equal to the center line
D. The process is stable because every point is within the limits

41 A process control chart shows no points beyond the control limits and no nonrandom patterns, but 6% of output falls outside the engineering specifications. Which conclusion is most defensible?

Introduction to statistical quality control Hard
A. The specifications should replace the computed control limits.
B. The process is capable but not statistically stable.
C. The control limits should be widened to include the specifications.
D. The process is stable but not necessarily capable.

42 A stable process contains only common-cause variation, but management wants lower variability. Which action is consistent with statistical quality control principles?

Introduction to statistical quality control Hard
A. Adjust the process after every deviation from the target.
B. Pursue system-level changes that reduce common-cause variation.
C. Recalculate the centerline after each new subgroup.
D. Inspect every unit and reset the machine after each observed deviation, even when no chart rule signals.

43 A process is stable with mean and standard deviation . Its specification limits are and . What are the approximate values of and ?

Process control Hard
A. and
B. and
C. and
D. and

44 For an independent normal process, a three-sigma chart has a probability of approximately that one in-control observation falls outside either limit. What is the probability that at least one false signal occurs among 25 observations?

Process control Hard
A.
B.
C.
D.

45 A process can drift gradually during an eight-hour shift. Which sampling design best creates rational subgroups that separate short-term variation from between-period drift?

Process control Hard
A. Measure five units selected randomly from the entire week.
B. Measure one unit hourly and combine eight hours as one subgroup.
C. Measure one unit whenever an operator suspects a process change.
D. Measure five consecutive units once every hour.

46 For subgroups of size , an - chart has , , , , and . A new subgroup has and . How should it be classified?

Control charts for variables: X and R charts Hard
A. Both the mean and range are out of control.
B. Both the mean and range are in control.
C. The mean is in control, but the range is out of control.
D. The mean is out of control, but the range is in control.

47 For subgroups of size , and . The process is centered at with specifications and . Using , which pair is approximately correct?

Control charts for variables: X and R charts Hard
A. and
B. and
C. and
D. and

48 An established - chart uses subgroups of size . Future subgroups will contain observations, while the underlying process variation is believed unchanged. What is the most defensible transition?

Control charts for variables: X and R charts Hard
A. Divide every old control limit by while retaining the previous range limits and centerline.
B. Change only while retaining the old range centerline.
C. Retain both charts because the physical process is unchanged.
D. Estimate from old data and construct limits using constants for .

49 An - chart for subgroups of size has , , , , and . A subgroup produces and . What does the chart indicate?

X and S charts Hard
A. Only the subgroup mean produces a control signal.
B. Only the subgroup standard deviation produces a control signal.
C. Both statistics are within their control limits.
D. Both statistics produce upper control-limit signals.

50 For an - chart with , suppose , , and . Using , what are the approximate -chart limits?

X and S charts Hard
A.
B.
C.
D.

51 An chart has , , and . Its software displays the UCL as , and a new subgroup has . What is correct if decisions use unrounded limits?

X and S charts Hard
A. It signals only if the corresponding mean also exceeds its UCL.
B. It does not signal because both displayed values equal .
C. It does not signal because an chart cannot have a zero LCL.
D. It signals because exceeds the exact UCL .

52 A variable-sample-size chart has . Sample 1 contains units with nonconforming; Sample 2 contains units with nonconforming. Using separate three-sigma limits for each sample, which statement is correct?

Control charts for attributes: p chart Hard
A. Both samples produce upper control signals.
B. Only Sample 2 produces an upper control signal.
C. Only Sample 1 produces an upper control signal.
D. Neither sample produces an upper control signal.

53 Three preliminary samples have sizes , , and , with , , and nonconforming units, respectively. What centerline should be used for the pooled chart?

Control charts for attributes: p chart Hard
A.
B.
C.
D.

54 A chart has and constant sample size . Its normal-approximation UCL is about . A sample contains two nonconforming units. The exact in-control probability of observing at least two is about . Which interpretation is best?

Control charts for attributes: p chart Hard
A. The point must be accepted as in control because every p chart with a zero LCL requires at least three nonconforming units before an upper signal can be declared.
B. The point signals, but the nominal three-sigma interpretation is inaccurate for such sparse data.
C. The point signals and its exact upper-tail probability is approximately .
D. The point does not signal because two nonconforming units are below three.

55 An chart uses constant samples of units and has . A new sample contains nonconforming units. Using three-sigma limits, what is the conclusion?

Control charts for attributes: np chart Hard
A. It is out of control because the UCL is approximately .
B. It is in control because the UCL is approximately .
C. It is in control because the UCL is approximately .
D. It is out of control because the UCL is approximately .

56 A process has historical fraction nonconforming . If an chart is constructed for a new constant sample size of , what are its approximate control limits?

Control charts for attributes: np chart Hard
A. and
B. and
C. and
D. and

57 Phase I data for a chart contain inspection units and total defects. One unit with defects is traced to an assignable cause and removed. Using the revised limits, how is a future unit with defects classified?

Control charts for attributes: c chart Hard
A. , ; the unit is in control.
B. , ; the unit signals.
C. , ; the unit signals.
D. , ; the unit is in control.

58 A chart has defects per inspection area. Every future inspection unit will have exactly twice the original area, and the defect rate per unit area is unchanged. What limits are appropriate under a homogeneous Poisson model?

Control charts for attributes: c chart Hard
A. and , centered at
B. and , centered at
C. and , centered at
D. and , centered at

59 At the same subgroup, an chart exceeds its UCL and the corresponding chart also signals. What is the best initial interpretation?

Applications of control charts in process control Hard
A. Ignore the range because the mean chart is always diagnostically superior.
B. Investigate the dispersion signal before concluding that the process mean shifted.
C. Conclude immediately that only the process mean has changed.
D. Recenter both charts at the signaling subgroup without investigating causes.

60 For an independent, continuous, in-control process centered on its chart centerline, what is the probability that a specified block of eight consecutive points all falls on the same side of the centerline?

Applications of control charts in process control Hard
A.
B.
C.
D.
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