This set of Statistical Quality Control Multiple Choice Questions & Answers (MCQs) focuses on “SPC Methods and Philosophy – Statistical Basis of the Control Chart – 1”.
1. The average value of the quality characteristic corresponding to in-control state is represented by _____ in the control charts.
d) Sample number
Explanation: The control chart contains a centre line (CL) that represents the average value of the quality characteristic corresponding to in-control state. This may be the desired value of that quality characteristic.
2. The highest value that a quality characteristic can take before the process becomes out-of-control, is called ______
a) Center line
b) Upper control limit
c) Lower control limit
d) Control limit
Explanation: The UCL is the highest value of a quality characteristic at which the process is in-control, i.e. it produces products of desired specifications.
3. The horizontal line in the control chart which shows the minimum value of a quality characteristic, before the process gets out-of-control, is called the _____
a) Upper control limit line
b) Lower control limit line
c) Desired value
d) Center line
Explanation: The lowest value for a Critical-to-quality characteristic corresponding to in-control state is shown on the control chart with the lowest horizontal line, which is called the lower control limit line.
4. In the horizontal axis of a control chart displays _______
a) Sample number
c) Either sample number or time
d) Neither sample number or time
Explanation: The horizontal axis of a control chart displays either sample numbers or time elapsed from a certain time or from the time of process starting.
5. If for a process, 18 out of 20 points are plotted above the CL but below the upper control limit, and only 2 of 20 are plotted between the center line and the lower control limit, what can we say about the process state?
c) Data is not enough to predict
d) Process state is not dependent over this data
Explanation: The above question concludes a systematic pattern. For a process to be perfectly in-control state, the plotted points on the corresponding control chart should have an essentially random pattern.
6. Shifting of mean from the current value to a new value and remaining there, is called _____ shift of the mean.
Explanation: Due to some reason, it may be possible that the process mean shifts from one point to another, and remain there for quite some time. This shifting of mean is called the sustained shift of the mean.
7. A process is said to be in the statistical control if it operates with ______ of variation.
a) Chance causes
b) Assignable causes
c) Both chance and assignable causes
d) Neither chance nor assignable causes
Explanation: Chance causes of variation produce the natural variability of the stable process. As this natural variability cannot be avoided, so a process is said to be in statistical control if it operates only with chance causes of variation.
8. Which of these is not a part of SPC’s 7 tools?
a) Pareto chart
c) Design of Experiments
d) Check sheet
Explanation: Pareto chart, check sheet, histogram, cause-and-effect diagram, scatter diagram, and control chart are all part of SPC (Statistical Process Control). Design of Experiments is not one of them.
9. If a process has its operation with _________ causes of variation, it is said to be an out-of-control process.
c) Neither chance nor assignable
Explanation: Only assignable causes of variation are able to shift the process from in-control to out-of-control state. So if a process possesses assignable causes of variation, it is stated to be out-of-control.
10. Chance causes of variation are also called ________ and assignable causes are also called _______
a) Common causes, special causes
b) Special causes, common causes
c) Variability causes, non-variability causes
d) Non-variability causes, variability causes
Explanation: Chance causes of variation, and assignable causes of variation are also called the common causes, and special causes of variation, respectively.
11. The chance and assignable cause terminology was developed by _______
Explanation: The chance and assignable cause terminology was developed by W. A. Shewhart. Now, common and special cause terminology is used.
12. OC curve can be used to determine the probability of _______
a) Type I error
b) Type II error
c) Both type I and type II error
d) Neither type I nor type II error
Explanation: OC curve for any process is a very useful tool to find out the probability of a type II error which means, concluding the process in-control when it is actually out-of-control.
13. Control charts can be used very effectively in the process of hypothesis testing.
Explanation: Control charts and hypothesis testing have a really close relation as control charts can be used to check if the assumptions of hypothesis testing hold good in the process operation.
14. Type I error is described as the situation of the conclusion of the process state as out-of-control when it is in-control.
Explanation: Type I error is actually concluding the process state being out-of-control when the process state is in-control. The opposite conclusion gives the conditions for Type II error.
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