What is defined by control limits?

Saw this question in Oliver Lehmann 75 questions.

What is defined by control limits?

1. A measuring instrument solely used to describe process capability. The process is found capable if the ±3 sigma range is exceeded by not more than 0.3% of a tested sample lot.

2. The limits of the six sigma area on either side of a control chart to plot measured values; data found outside the area are out of specification and can lead to rejection of an entire batch.

3. The area consisting of typically three standard deviations on either side of a mean value of a control chart to plot measured values found in statistical quality control

4. The area consisting of typically three standard deviations on either side of a mean value of a control chart to plot measured values found in quality assurance 

The correct answer is 3.

Why answer is not 4 because control limits are used in both quality control and quality assurance?

This is not a PMI pattern question . It is highly atypical and exeptional.

Control limit in general is associated with control chart.

While +/- 3sigma , range is associated with PERT estimate technique.

where as in quality concerned 6 sigma approach, consider 6 sigma range.

So this question is meshed with a hypothetic flaws.

Hi,


First: Why answer is not 4 ?. You are right, control charts are used both in Quality assurance and Quality Control. But only thing to understand is why and how?


Values are typically measured and plotted on control charts in Quality Control and the output is called "Quality Measurements" and these "Quality Measurements" become input for Quality assurance (Please check ITTO of both process). Now in QA you typically check or audit whether the process you are following is as per the organization policies and standards. Normally here you dont measure but audit your control chart or the values from it. It may be like "Are the control charts using the right Control and Specification limits", "Are rule of seven taken care of" or it might be used as a tool for "recording the number of non-conformances". So essentially when both QC and QA are the options for control chart, QC takes precedence as Control charts are typical of QC. (Its like "can you record video with a digital camera and a HandyCam" obviously Handycam takes precedence)


Now among the answers there is mention of Six sigma also. Though six sigma is a measurement limit, it is more to measure the "Process capability" of producing defect free products. It says 3.4 defects per Million oppurtunities. This essentially does not mean, if you find 4 defective pieces among one million pieces you will reject the entire batch of one million(as in the answer). This only says that the process can be still improved to bring it under 4.


So among the answers given, 3 option is the best as it represents a typical control chart characteristic.


Hope this helps.