Up to three standard deviations above or below the centerline is the amount of variation that statistical process control allows for:
A) Type I errors.
B) about 95.5% variation.
C) natural variation.
D) all types of variation.
E) assignable variation.

Question

Quizplus · Accepted Answer

This is known as the control limits or the range of natural variation in a process. It allows for a certain amount of variation that is inherent in the process, while flagging any data points that fall outside of this range as potentially indicating an issue with the process that needs to be addressed. Choice A is incorrect because Type I errors refer to errors made in hypothesis testing, and are not directly related to statistical process control. Choice B is incorrect because while 95.5% of data falls within three standard deviations of the mean in a normal distribution, this does not necessarily apply to all processes or forms of data. Choice D is incorrect because statistical process control does not allow for all types of variation – for example, any variation due to external factors that is not inherent to the process being measured should be eliminated or controlled for. Choice E is incorrect because assignable variation refers to fluctuations in a process that can be traced back to specific causes, and should be addressed rather than simply allowed for.

Up to Three Standard Deviations Above or Below the Centerline