Question 1

Small haphazard changes in a process variable due to alteration in the production environment that is not controllable are said to be assignable causes.

Accepted Answer

Small haphazard changes in a process variable that are not controllable are said to be due to common causes or natural variation, while assignable causes are typically associated with larger, more significant changes that can be traced back to specific factors or events in the production process that are controllable.

Question 2

For a  control chart, the lower and upper control limits are usually set at:
A) one standard deviation from the centerline
B) two standard deviations from the centerline
C) three standard deviations from the centerline
D) four standard deviations from the centerline
E) five standard deviations from the centerline

Accepted Answer

Using three standard deviations from the centerline ensures that 99.7% of the observations fall within the control limits, providing a reasonable assurance that the process is in control. Choosing two or four standard deviations would either reduce or increase the stringency of the control limits, respectively. Choosing one or five standard deviations would either be too lenient or too impractical, respectively.

Question 3

If a process is in control, we expect all the data values to fall within three standard deviations of the mean.

Accepted Answer

This statement is true according to the statistical quality control concept of the "3-sigma rule." If a process is in control, we expect about 99.7% of the data points to fall within three standard deviations (3-sigma) of the mean.

Question 4

Once a process is in control and is producing a satisfactory product, the process variables are monitored by use of control charts.

Accepted Answer

This statement is true. Control charts are used to monitor process variables to ensure that the process remains in control and is producing consistent, satisfactory results.

Question 5

The mean of the sample means and the standard deviation of 50 samples of size 5 taken from a production process under control are found to be 300 and 25, respectively. The lower control limit for the  chart is located at:
A) 333.54
B) 289.39
C) 310.61
D) 266.46
E) 342.88

Accepted Answer

The answer of The mean of the sample means and...

Question 6

The first objective in statistical process control is to eliminate assignable causes of variation in the process variable and then get the process in control. The next step is to reduce variation and get the measurements on the process variable within specification limits, the limits within which the measurements on usable items or services must fall.

Accepted Answer

This statement accurately describes the first two objectives of statistical process control.

Question 7

If a process is in control, we expect all the data values to fall within two standard deviations of the mean.

Accepted Answer

In a process that is in control, we expect about 95% of the data values to fall within two standard deviations of the mean, but we typically use three standard deviations from the mean to account for 99.73% of the data, which is a more common standard for determining control.

Question 8

Assignable cause variation refers to variation in the output of a process that is naturally occurring, expected, and that may be the result of random causes.

Accepted Answer

This statement is actually describing common cause variation, not assignable cause variation. Assignable cause variation refers to variation that is caused by specific, identifiable factors that can be addressed and eliminated to improve process output.

Question 9

Random cause variation refers to variation n, the output of a process that is unexpected and has an assignable cause.

Accepted Answer

Random cause variation refers to variation in the output of a process that is expected and has no assignable cause. It is also known as common cause variation.

Question 10

Fifty samples of size 500 were drawn from a manufacturing process and the number of defectives in each sample was counted. The average of the sample proportion was 0.032. The centerline for the p chart is:
A) 0.032
B) 16.0
C) 0.512
D) 0.968
E) 4.2

Accepted Answer

The centerline of a p chart represents the average proportion of defectives in the samples, which is given as 0.032.

Question 11

Twenty five samples of size 1000 each were drawn from a manufacturing process and the number of defectives in each sample was counted. The average sample proportion was 0.05. The upper control limit for the p chart is:
A) 0.0475
B) 0.0206
C) 0.0293
D) 0.0707
E) 0.0635

Accepted Answer

The upper control limit (UCL) for a p-chart is calculated using the formula: UCL = $ \bar{p} + Z\sqrt{\frac{\bar{p}(1-\bar{p})}{n}} $, where $ \bar{p} $ is the average proportion of defectives, $ Z $ is the Z-value from the standard normal distribution corresponding to the desired confidence level (commonly 3 for control charts, representing 3 standard deviations from the mean), and $ n $ is the sample size. Given $ \bar{p} = 0.05 $ and $ n = 1000 $, and assuming a Z-value of 3 for control charts, the calculation is: UCL = 0.05 + 3*sqrt((0.05*0.95)/1000) = 0.05 + 0.0207 = 0.0707.

Question 12

Random variation in a process can be eliminated.

Accepted Answer

Random variation, also known as noise or error, is inherent in any process and cannot be completely eliminated. It can only be reduced by improving the process or using control measures.

Question 13

Process control charts, such as the  and p-charts, are used to provide signals to indicate when the output of a process is out of control.

Accepted Answer

Process control charts are specifically designed to detect when a process is out of control and provide signals for corrective action. The 11eb6b9e_0657_4754_8d9d_156bd8245371_TB8220_11 and p-charts are examples of control charts commonly used for this purpose.

Question 14

Statistical process control (SPC) methodology was developed to monitor, control, and improve products and services.

Accepted Answer

Statistical process control (SPC) methodology is a quality control tool that was developed to monitor, control, and improve products and services. It uses statistical analysis to identify causes of variation in a process, and then allows for adjustments to be made to bring the process back to its desired level of performance. SPC has been widely used in industry and manufacturing, but can also be applied to service industries such as healthcare and finance.

Question 15

If a process is in control, we expect all the data values to fall within one standard deviation of the mean.

Accepted Answer

In a process that is in control, we typically expect about 68% of the data values to fall within one standard deviation of the mean, about 95% within two standard deviations, and about 99.7% within three standard deviations, according to the empirical rule for a normal distribution.

Question 16

In most processes, the process control limits are set to correspond with the specification limits on the process.

Accepted Answer

Process control limits are set based on the performance of the process, whereas specification limits are the allowable range for a particular characteristic or feature of the product or service being produced. Process control limits need to be within the specification limits, but they may not necessarily be set to correspond exactly with them. It is possible for a process to be within control limits but still produce products that fall outside of specification limits.

Question 17

The cause of a change in a process variable being monitored is regarded as random variation if it can be found and corrected.

Accepted Answer

Random variation refers to variation that is inherent in the process and cannot be fully eliminated. It is not possible to always find and correct the cause of a change in process variable that is due to random variation.

Question 18

One of the most common sources of random cause variation is the people who are working in the process.

Accepted Answer

People are known to have a degree of variability in their work habits, skills, experience, etc. This can result in random cause variation in the process.

Question 19

In control process is typically defined as a process in which all output is operating within  3 standard deviation of the centerline of the process.

Accepted Answer

This statement is true. In statistical process control, a process is said to be in control if all outputs are operating within 3 standard deviations of the mean or centerline of the process. This is often represented by a control chart, which helps to monitor and maintain the stability of the process.

Question 20

If the variation in a process variable is solely random, the process is said to be in control.

Accepted Answer

If the variation in a process variable is solely random, and there are no non-random sources of variation or special causes present, the process is considered to be in control. This means that the process is stable and predictable within its statistical limits, and any deviations from the mean or target value are due to common causes of variation.

Quiz 7: Sampling Distributions

Small haphazard changes in a process variable due to alteration in the production environment that is not controllable are said to be assignable causes.

For a control chart, the lower and upper control limits are usually set at: