3.2 Stability and Capability Flashcards
In Statistical Process Control, define the center line, UCL, and LCL
the center line is the middle of the data, usually the mean. Upper (UCL) and Lower (LCL) control limits show the edges of normal variation, usually +/- 3 standard deviations.
What is the difference between control limits and specification limits?
Control limits are determined by the process. Specification limits are determined by the customer.
In control charting what is subgrouping and what is staging?
Subgrouping is combining data points to reduce noise. i.e. charting a weekly average instead of a daily reading.
Staging is recalculating control limits based on a significant change in the process. i.e. after a process improvement.
What is the difference between common cause and special cause variation?
Common cause variation is random noise in the data with no known cause.
Special cause variation is a non-random signal in the data with a known (or discoverable) cause.
Why do we chart both output readings and ranges in paired control charts like IMR or xbar S charts?
A large movement between points can indicate a process shift, even if both points are in control.
What are the 4 most common run rules for SPC charts
Any point more than 3 standard deviations from the mean.
9 points in a row on the same side of the center line.
6 points in a row all increasing or decreasing.
14 points in a row alternating up and down.
What are the 3 most common data distribution patterns
Normal
Bi-Modal
Uniform
What is cpk and what is a general standard for a good cpk?
Process capability index. It shows how well a process fits within the customer’s specifications. A cpk of 1.66 or higher shows a capable process.
How are cpk, sigma levels, and ppm defective related?
They are all different ways of showing the same thing, how likely a process is to match customer specifications.
