This set of Tough Statistical Quality Control Questions and Answers focuses on “Statistical Process Control for Short Production Runs – 4”.
1. EWMA and cusum charts can be used for the short production runs because ____
a) They are good in phase II
b) They have shorter ARL
c) They don’t detect process shift
d) They detect larger process shift
Explanation: EWMA charts and the cusum charts both, find applications in the short production run cases. This is due to the fact that the EWMA charts and the Cusum charts both have shorter ARL than the Shewhart charts.
2. Which of these is suitable for the short run environment?
a) x bar chart
b) R chart
d) Self starting cusum chart
Explanation: The Cusum charts are effective for subgroups size of one. The self starting version of the Cusum charts is particularly suitable for the short run environment due to the same fact.
3. Which of these is not one of the areas for which, the self starting cusum approach uses regular process measurements for?
a) For establishing the cusum
b) For calibrating the cusum
c) For acceptance sampling
d) For process monitoring
Explanation: The self starting approach of cusum charts uses regular process measurements for both establishing the cusum, and calibrating of cusum. These measurements are also used for process monitoring.
4. Which of these is eliminated by the self starting approach?
a) Phase I parameter estimation phase
b) Phase I parameter measurement phase
c) Phase II parameter estimation phase
d) Phase II parameter measurement phase
Explanation: The self starting approach uses regular process measurements for establishing and calibrating the cusum, and for process monitoring. Thus, this eliminates the phase I parameter estimation stage.
5. The false alarm rate is ___ when small number of subgroups is used for the Shewhart control charts.
c) Remains same
d) Changed randomly
Explanation: The number of subgroups used in calculating the trial control limits for Shewhart control charts, impacts the false alarm rate of the chart. This rate is inflated when small number if subgroups is used.
6. Who was the first person to study the effect of the number of subgroups used for x bar and R chart control limit calculation on the false alarm rate?
Explanation: The false alarm rate of the Shewhart control charts also depends over the number of subgroups used to construct the trial control limits. This was first studied by Hillier (1969).
7. Which of these charts can be used instead of Shewhart control charts, in the case of small number of subgroups used to construct the charts, which were recommended by Quesenberry?
Explanation: Quesenberry recommended the Q charts to be used in the place of the Shewhart control charts, in the case of small subgroup number used to calculate the control limits. This was because small number of subgroups used with Shewhart control charts, increased their FAR.
8. Which method was suggested by Del Castillo and Montgomery, to be used instead of the Q-chart to get better ARL performance?
a) Kalman Filter
b) Acceptance Sampling
c) Design of experiments
d) Deleting some samples completely
Explanation: Del Castillo and Montgomery found the ARL performance of the Q-charts very inadequate. So, they recommended the Kalman filter that has a better ARL performance than Q-chart.
9. Which of these is not a purpose for which control charts are produced for?
a) Control of the process
b) Reduction of variability
c) Increment of variability
d) Continuous process improvement
Explanation: The control charts are generally plotted for the main reasons like control of the process or statistical monitoring, reduction of variability, and continuous process improvement.
10. Which of these is used when the process has achieved high capability level?
a) Trial control limits
b) Actual control limits
c) Modified control limits
d) Easy control limits
Explanation: At the stage, when the process has achieved a high level of capability, there is some relaxation provided to the level of the surveillance provided. This is generally done with modified control limits in the case of x charts.
11. Which of these is not a name of Modified control limits for x̅ charts?
a) Acceptance limits
b) Reject limits
c) Reserve limits
d) Liquid limits
Explanation: The relaxation in the case of x charts in the case of high process capability is provided with the use of the modified limits. These are also called reject limits.
12. Which of these is a method used when the process capability is quite high?
a) Acceptance sampling
b) Design of experiments
c) Acceptance control charts
d) Shewhart control charts
Explanation: When the process capability becomes very high, the relaxation of the level of surveillance is provided by using two methods; first is the reject limits, and the second is the acceptance control chart.
13. Which of these does not say that the modified control limits must be applied?
b) Process spread is considerably small
c) Natural variability becomes high
d) Process capability becomes quite good
Explanation: The modified control limits are used when the process capability is very high. The PCR Cpk≫1, very less process spread, and low natural variability indicate the high process capability. So, modified control charts are used in these conditions.
14. The six-sigma approach when implemented for a long time, the process may need modified control charts.
Explanation: As six-sigma approach to variability reduction focuses on improving process until the minimum value of Cpk=2. So this means high process capability, which encourages modified control charts usage.
15. If the Cpk=0.9899, the usage of modified control charts is very necessary.
Explanation: The modified control charts have the use of the modified control limits, which are used when the process capability is high or Cpk≫1. As Cpk=0.9899, the usage of modified control charts is not necessary.
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