Operations Management - Chapter 10
Quality control, in contrast to quality assurance, is implemented: A. after production. B. before inspection. C. by self-directed teams. D. by top management. E. during production.
E. during production.
A c-chart is used for: A. means B. ranges C. percent defective D. fraction defective per unit E. number of defects per unit
E. number of defects per unit C-charts monitor the number of defects per unit.
Which of the following quality control sample statistics indicates a quality characteristic that is an attribute? A. mean B. variance C. standard deviation D. range E. proportion
E. proportion Proportions would be control with attribute control charts.
A c-chart is used to monitor the total number of defectives in the output of a process.
FALSE A c-chart is used to monitor the number of defects per unit, not defective units.
Approving the effort that occurs during the production process is known as acceptance sampling.
FALSE Acceptance sampling occurs before or after the production process.
A process that exhibits random variability would be judged to be out of control.
FALSE All processes exhibit random variability.
An "up and down" run test uses the median as a reference point and measures the percentage above and below the median.
FALSE An up-and-down runs test looks only at runs of increasing or decreasing values.
Approximately 99.7% of sample means will fall within ± two standard deviations of the process mean if the process is under control.
FALSE Approximately 99.7% of sample means will fall within ± three standard deviations of the process mean.
Attributes need to be measured, variable data can be counted.
FALSE Attributes need to be counted, variable data is measured.
Control limits are based on multiples of the process standard deviation.
FALSE Control limits are based on multiples of the standard deviation of the sample statistic.
Control limits used on process control charts are specifications established by design or customers.
FALSE Control limits are independent of specifications.
The process capability index (indicated by Cpk) can be used only when the process is centered.
FALSE Cpk can be used whether or not the process is centered.
The greater the capability ratio, the higher the rejects.
FALSE Greater capability reduces rejects.
The larger the process variation, the tighter the specifications should be.
FALSE Greater variation would lead to wider specifications.
Non-random variation is likely whenever all observations are between the LCL and UCL.
FALSE If all observations are between the LCL and UCL, then the process would be considered in control.
Even if the process is not centered, the process capability index (indicated by Cpk) is very useful.
FALSE If the process is not centered, Cpk is not useful.
An R value of zero (on a range chart) means that the process must be in control since all sample values are equal.
FALSE If the sample size is sufficiently large, an R of zero could indicate an out of control process.
Processes that are in control eliminate variations.
FALSE In control, processes are free of non-random variation.
The purpose of statistical process control is to ensure that historical output is random.
FALSE It is to ensure that non-random variation is detected and corrected.
Larger samples will require wider x-bar control limits because there is more data.
FALSE Large samples will lead to narrower control limits.
Range charts and p-charts are both used for variable data.
FALSE P-charts are used with attribute data.
Range charts are used mainly with attribute data.
FALSE Range charts are used with variable data.
Range control charts are used to monitor process central tendency.
FALSE Ranger charts monitor variability.
Run tests give managers an alternative to control charts; they are quicker and cost less.
FALSE Runs tests are not alternatives to control charts.
Statistical Process Control is the measurement of rejects in the final product.
FALSE SPC is the evaluation of the process.
Statistical process control focuses on the acceptability of process output.
FALSE Statistical process control focuses on the variability of processes.
The Taguchi Cost Function suggests that the capability ratio can be improved by extending the spread between LCL and UCL.
FALSE The Taguchi cost function suggests that reducing variation is key.
The best way to assure quality is to use extensive inspection and control charts.
FALSE The best way to assure quality is to make sure processes are highly capable.
The number of defective parts in a sample is an example of variable data because it will "vary" from one sample to another.
FALSE The number of defective parts in a sample is an example of attribute data.
The optimum level of inspection occurs when we catch at least 98.6 percent of the defects.
FALSE The optimum level of inspection is when the sum of inspection costs and the cost of passing defectives are equal.
The primary purpose of statistical process control is to detect a defective product before it is shipped to a customer.
FALSE The primary purpose of SPC is to detect nonrandomness.
An x-bar control chart can only be valid if the underlying population it measures is a normal distribution.
FALSE The sample average typically is normally distributed regardless of the underlying distribution of the process.
Attribute data are counted, variable data are measured.
TRUE These distinguish attribute from variable data.
The amount of inspection needed is governed by the costs of inspection and the expected costs of passing defective items.
TRUE These interact to set the optimum amount of inspection.
The sampling distribution can be assumed to be approximately normal even when the underlying process distribution is not normally distributed.
TRUE This is especially true as the sample size grows.
The optimum level of inspection minimizes the sum of inspection costs and the cost of passing defectives.
TRUE This represents the optimum balance between inspection and failure costs.
Variation in a sample statistic collected from a process may be either random variation or assignable variation - or both.
TRUE Total variation can consist of both random and assignable variation.
Inspection is a(n): A. prevention. B. control. C. monitoring. D. corrective. E. appraisal.
E. appraisal.
A process results in a few defects occurring in each unit of output. Long-run, these defects should be monitored with ___________. A. p-charts B. c-charts C. x-bar charts D. r-charts E. o-charts
B. c-charts C-charts are used to monitor the number of defects per unit.
The more effective and all-encompassing a firm's quality control and continuous improvement efforts, the less that company will need to rely on: A. insourcing. B. inspection. C. outsourcing. D. acceptance sampling. E. capability assessment.
B. inspection.
_______ variation is a variation whose cause can be identified. A. Assignable B. Controllable C. Random D. Statistical E. Theoretical
A. Assignable Assignable variation has a special cause.
A control chart used to monitor the fraction of defectives generated by a process is the: A. p-chart B. R-chart C. x-bar chart D. c-chart E. Gantt chart
A. p-chart The p-chart monitors the fraction defective.
Studies on a machine that molds plastic water pipe indicate that when it is injecting 1-inch diameter pipe, the process standard deviation is 0.05 inches. The one-inch pipe has a specification of 1-inch plus or minus 0.10 inch. What is the process capability index (Cpk) if the long-run process mean is 1 inch? A. 0.50 B. 0.67 C. 1.00 D. 2.00 E. none of the above
B. 0.67 Use the Cpk formula to assess this process' capability.
The specifications for a product are 6 mm ± 0.1 mm. The process is known to operate at a mean of 6.05 with a standard deviation of 0.01 mm. What is the Cpk for this process? A. 3.33 B. 1.67 C. 5.00 D. 2.50 E. none of the above
B. 1.67 Cpk is used here since the process mean isn't centered in the specification interval.
The basis for a statistical process control chart is a(the) __________. A. process capability B. sampling distribution C. control limit D. sample range E. sample mean
B. sampling distribution Control charts reflect the sampling distribution of an in control process.
A shift in the process mean for a measured characteristic would most likely be detected by a: A. p-chart B. x-bar chart C. c-chart D. R-chart E. s-chart
B. x-bar chart X-bar charts monitor the process mean.
The specification limit for a product is 8 cm and 10 cm. A process that produces the product has a mean of 9.5 cm and a standard deviation of 0.2 cm. What is the process capability, Cpk? A. 3.33 B. 1.67 C. 0.83 D. 2.50 E. none of the above
C. 0.83 Cpk is used here since the process mean isn't centered in the specification interval.
A time-ordered plot of sample statistics is called a(n) ______ chart. A. Statistical B. Inspection C. Control D. SIMO E. Limit
C. Control A control chart is a time-ordered plot of sample statistics.
A time-ordered plot of representative sample statistics is called a: A. Gantt chart B. SIMO-chart C. Control Chart D. Up-Down Matrix E. Standard deviation table
C. Control Chart Control charts are time-ordered plots of sample statistics.
If a process is performing as it should, it is still possible to obtain observations which are outside of which limits? (I) tolerances (II) control limits (III) process variability A. I B. II C. I and II D. II and III E. I, II, and III
C. I and II Even capable, in control processes can have observations outside of control limits or tolerances.
A plot below the lower control limit on the range chart: (I) should be ignored since lower variation is desirable (II) may be an indication that process variation has decreased (III) should be investigated for assignable cause A. I and II B. I and III C. II and III D. II only E. I, II, and III
C. II and III Plots outside of control limits should be investigated.
Which of the following relationships must always be incorrect? A. Tolerances > process variability > control limits B. Process variability > tolerances > control limits C. Tolerances > control limits > process variability D. Process variability > control limits > tolerances E. Process variability <Tolerances<control limits
C. Tolerances > control limits > process variability Process variability will always be greater than control limits.
Which of the following is not a step in the quality control process? A. define what is to be controlled B. compare measurements to a standard C. eliminate each of the defects as they are identified D. take corrective action if necessary E. evaluate corrective action
C. eliminate each of the defects as they are identified Eliminating defects is not part of quality control.
A point which is outside of the lower control limit on an R-chart: A. is an indication that no cause of variation is present B. should be ignored because it signifies better than average quality C. should be investigated because an assignable cause of variation might be present D. should be ignored unless another point is outside that limit E. is impossible since the lower limit is always zero
C. should be investigated because an assignable cause of variation might be present Points outside of the control limits should be investigated as signals of non-random variation being present.
A p-chart would be used to monitor _______. A. average shrinkage B. dispersion in sample data C. the fraction defective D. the number of defects per unit E. the range of values
C. the fraction defective The p-chart monitors the fraction defective.
A control chart used to monitor the process mean is the: A. p-chart B. R-chart C. x-bar chart D. c-chart E. Gantt chart
C. x-bar chart The x-bar chart monitors the process mean.
When a process is in control, it results in there being, on average, 16 defects per unit of output. C-chart limits of 4 and 28 would lead to a _______ chance of a Type I error. A. 67% B. 92% C. 33% D. 0.3% E. 5%
D. 0.3% These would be three-sigma limits.
Studies on a bottle-filling machine indicates it fills bottles to a mean of 16 ounces with a standard deviation of 0.10 ounces. What is the process specification, assuming the Cpk index of 1? A. 0.10 ounces B. 0.20 ounces C. 0.30 ounces D. 16.0 ounces plus or minus 0.30 ounces E. none of the above
D. 16.0 ounces plus or minus 0.30 ounces Use the Cpk formula to solve for the specification interval.
The probability of concluding that assignable variation exists when only random variation is present is: (I) the probability of a Type I error (II) known as the alpha risk (III) highly unlikely (IV) the sum of probabilities in the two tails of the normal distribution A. I and II B. I and IV C. II and III D. I, II, and IV E. I, III, and IV
D. I, II, and IV Incorrect signals can be on either side of the distribution.
A control chart used to monitor the number of defects per unit is the: A. p-chart B. R-chart C. x-bar chart D. c-chart E. Gantt chart
D. c-chart C-charts monitor the number of defects per unit.
The purpose of control charts is to: A. estimate the proportion of output that is acceptable B. weed out defective items C. determine if the output is within tolerances/specifications D. distinguish between random variation and assignable variation in the process E. provide meaningful work for quality inspectors
D. distinguish between random variation and assignable variation in the process Control charts are used to signal assignable variation.
The optimum level of inspection is where the: A. cost of inspection is minimum B. cost of passing defectives is minimum C. total cost of inspection and defectives is maximum D. total cost of inspection and defectives is minimum E. difference between inspection and defectives costs is minimum
D. total cost of inspection and defectives is minimum At the optimum level these costs are, in total, minimized.
Organizations should work to improve process capability so that quality control efforts can become more ________. A. effective B. efficient C. necessary D. unnecessary E. widespread
D. unnecessary Increasing process capability reduces the necessity for quality control.
The range chart (R-chart) is most likely to detect a change in: A. proportion B. mean C. number defective D. variability E. sample size
D. variability The range chart monitors variability.
When a process is in control, it results in there being, on average, 16 defects per unit of output. C-chart limits of 8 and 24 would lead to a _______ chance of a Type I error. A. 67% B. 92% C. 33% D. .03% E. 5%
E. 5% These would be two-sigma limits
The process capability index (Cpk) may mislead if: (I) the process is not stable. (II) the process output is not normally distributed. (III) the process is not centered. A. I and II B. I and III C. II and III D. II only E. I, II and III
E. I, II and III When using Cpk these concerns should be addressed.
Low-cost, high-volume items often require more intensive inspection.
FALSE These are not good candidates for inspection.
Type I and Type II errors refer to the magnitude of variation from the standard.
FALSE These refer to decisions regarding whether the process is in or out of control.
Tolerances represent the control limits we use on the charts.
FALSE Tolerances are specification limits, not control limits.
Concluding a process is out of control when it is not is known as a Type I error.
TRUE A Type I error involves erroneously concluding that a process is out of control.
A c-chart is used to monitor the number of defects per unit for process output.
TRUE A c-chart monitors the number of defects per unit for process output.
If a point on a control chart falls outside one of the control limits, this suggests that the process output is non-random and should be investigated.
TRUE A point outside the control limits suggests non-random variation.
The output of a process may not conform to specifications even though the process may be statistically "in control."
TRUE A process can be free of non-random variation and still not meet specifications.
"Assignable variation" is variation due to a specific cause, such as tool wear.
TRUE Assignable variation is specific cause variation.
Quality control is assuring that processes are performing in an acceptable manner.
TRUE Control is used to monitor the performance of processes.
When a process is not centered, its capability is measured in a slightly different way. The symbol for this case is Cpk.
TRUE Cpk is used when the process is not centered.
Patterns of data on a control chart suggest that the process may have non-random variation.
TRUE Ideally, the data on a control chart will have no pattern.
A p-chart is used to monitor the fraction of defectives in the output of a process.
TRUE P-charts involve the fraction of defectives.
"Process capability" compares "process variability" to the "tolerances."
TRUE Process variability influences how much output falls outside of tolerances.
Control limits tend to be wider for more variable processes.
TRUE Process with inherently more variability will naturally have wider control limits.
Run tests are useful in helping to identify nonrandom variations in a process.
TRUE Runs tests are useful to identify non-randomness in patterns.
A run test checks a sequence of observations for randomness.
TRUE Runs tests can be used to detect nonrandomness in sequences of observations.
"Quality of conformance" is concerned with whether a product or service conforms to its specifications.
TRUE Specification conformance is quality of conformance.
A lower control limit must by definition be a value less than an upper control limit.
TRUE The lower limit must be smaller than the upper limit.
The variation of a sampling distribution is tighter than the variation of the underlying process distribution.
TRUE The sampling distribution exhibits less variation than the underlying process.
High-cost, low-volume items often require careful inspection since we make them so infrequently.
TRUE These are good candidates for inspection.
The amount of inspection we choose can range from no inspection at all to inspecting each item numerous times.
TRUE These are the extremes of inspection.
