If a sample of items is taken and the mean of the sample is outside the control limits the process is:
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out of control and the cause should be established
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The causes of variation in statistical process control are:
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natural causes and assignable causes
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Natural variations:
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a. affect almost every production process
b. are the many sources of variation that occur when a process is under control
c. when grouped, form a pattern, or distribution
d. are tolerated, within limits, when a process is under control
e. All of the above are true.
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Natural variations:
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lead to occasional false findings that processes are out of control
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Assignable variation:
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is to be identified and eliminated
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Assignable causes:
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are causes of variation that can be identified and removed
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Control charts for variables are based on data that come from:
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averages of small samples
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The purpose of an X chart is to determine whether there has been a:
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change in the central tendency of the process output
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Statistical process control charts:
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display upper and lower limits for process variables or attributes, and signal when a process is no longer in control
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A sample of parts is measured. The mean of this sample is in the middle of the control limits, but some individual parts measure too low for design specifications and other parts measure too high. Which of the following is true?
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The process is in control, but not capable of producing within the established control limits
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The Central Limit Theorem:
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allows managers to use the normal distribution as the basis for building some control charts
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For an x-bar chart where the standard deviation is known, the Upper Control Limit:
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is 3 n above the mean of sample means for a 3 control chart
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Up to three standard deviations above or below the centerline is the amount of variation that statistical process control allows for:
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natural variation
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A manager wants to build 3 control limits for a process. The target value for the mean of the process is 10 units, and the standard deviation of the process is 6. If samples of size 9 are to be taken, the UCL and LCL will be:
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16 and 4
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The type of inspection that classifies items as being either good or defective is:
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attribute inspection
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The x-bar chart tells us whether there has been a:
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change in the central tendency of the process output
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The mean and standard deviation for a process for which we have a substantial history are x = 120 and = 2. For the variable control chart, a sample size of 16 will be used. What is the mean of the sampling distribution?
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none of the above
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Jars of pickles are sampled and weighed. Sample measures are plotted on control charts. The ideal weight should be precisely 11 oz. Which type of chart(s) would you recommend?
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x - and R-charts
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If x = 23 ounces, = 0.4 ounces, and n = 16, the 3 control limits will be:
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22.70 to 23.30 ounces
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The usual purpose of an R-chart is to signal whether there has been a:
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gain or loss in dispersion
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A manager wishes to build a 3 range chart for a process. The sample size is five, the mean of
sample means is 16.01, and the average range is 5.3. From Table S6.1, the appropriate value of D3
is 0, and D4 is 2.115. The UCL and LCL for this range chart are:
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11.2 and 0
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Plots of sample ranges indicate that the most recent value is below the lower control limit. What course of action would you recommend?
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Variation is not in control; investigate what created this condition
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To set x -chart upper and lower control limits, one must know the process central line, which is the:
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average of the sample means
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According to the text, the most common choice of limits for control charts is usually:
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± 3 standard deviations
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Which of the following is true of a p-chart?
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The lower control limit may be at zero
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The normal application of a p-chart is in:
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process sampling by attributes
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The statistical process chart used to control the number of defects per unit of output is the:
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c-chart
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The c-chart signals whether there has been a:
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change in the number of defects per unit
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The local newspaper receives several complaints per day about typographic errors. Over a seven day period, the publisher has received calls from readers reporting the following number of errors: 4, 3, 2, 6, 7, 3, and 9. Based on these data alone, what type of control chart(s) should the publisher use?
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c-chart
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A manufacturer uses statistical process control to control the quality of the firm's products. Samples of 50 of Product A are taken, and a defective/acceptable decision is made on each unit sampled. For Product B, the number of flaws per unit is counted. What type(s) of control charts should be used?
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p-chart for A, c-chart for B
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A nationwide parcel delivery service keeps track of the number of late deliveries (more than 30 minutes past the time promised to clients) per day. They plan on using a control chart to plot their results. Which type of control chart(s) would you recommend?
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c-charts
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A run test is used:
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to examine points in a control chart to check for nonrandom variability
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Which of the following is true regarding the process capability index Cpk?
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The larger the Cpk, the more units meet specifications
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If the Cpk index exceeds 1:
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must be less than one-third of the difference between the specification and the process mean
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The statistical definition of Six Sigma allows for 3.4 defects per million. This is achieved by a Cpk index of:
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2
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A Cpk index of 1.00 equates to a defect rate of:
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2.7 per 1,000 items
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Acceptance sampling:
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is used to determine whether to accept or reject a lot of material based on the evaluation of a
sample
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Acceptance sampling's primary purpose is to:
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decide if a lot meets predetermined standards
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An acceptance sampling plan's ability to discriminate between low quality lots and high quality lots is described by:
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an operating characteristics curve
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Acceptance sampling:
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a. may involve inspectors taking random samples (or batches) of finished products and measuring
them against predetermined standards
b. may involve inspectors taking random samples (or batches) of incoming raw materials and
measuring them against predetermined standards
c. is more economical than 100% inspection
d. may be either of a variable or attribute type, although attribute inspection is more common in
the business environment
e. All of the above are true.
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Which of the following statements on acceptance sampling is true?
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a. Acceptance sampling draws samples from a population of items, tests the sample, and accepts
the entire population if the sample is good enough, and rejects it if the sample is poor enough.
b. The sampling plan contains information about the sample size to be drawn and the critical
acceptance or rejection numbers for that sample size.
c. The steeper an operating characteristic curve, the better its ability to discriminate between good
and bad lots.
d. All of the above are true
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Acceptance sampling is usually used to control:
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incoming lots of purchased products
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An operating characteristic (OC) curve describes:
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how well an acceptance sampling plan discriminates between good and bad lots
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An operating characteristics curve shows:
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how the probability of accepting a lot varies with the population percent defective
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Producer's risk is the probability of:
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rejecting a good lot
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Which of the following is true regarding the relationship between AOQ and the true population percent defective?
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AOQ is less than the true percent defective
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Average outgoing quality (AOQ) usually:
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improves with inspection
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A Type I error occurs when:
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a good lot is rejected
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A Type II error occurs when:
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a bad lot is accepted
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In most acceptance sampling plans, when a lot is rejected, the entire lot is inspected and all defective items are replaced. When using this technique the AOQ:
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improves (AOQ becomes a smaller fraction)
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An acceptance sampling plan is to be designed to meet the organization's targets for product quality and risk levels. Which of the following is true?
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AQL, LTPD, and collectively determine n and c.
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A lot that is accepted by acceptance sampling:
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a. has more defects than existed before the sampling
b. has had all its defects removed by 100% inspection
c. will have the same defect percentage as the LTPD
d. has no defects present
e. All of the above are false
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Which of the following statements about acceptance sampling is true?
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The steeper an OC curve, the better it discriminates between good and bad lots
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Which of the following is true regarding the average outgoing quality level?
answer
AOQ is very low (very good) for extremely poor quality lots
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