Example of accepting or rejecting a lot based on a sample
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You have implemented a variables sampling plan to verify the pipe wall thickness of incoming 2" pipe. The lower specification for the wall thickness of the piping is 0.09". You and the supplier agree that the AQL is 100 defectives per million and the RQL is 300 defectives per million. Based on these specifications, you have randomly selected and measured 104 pipes of the 2500 per lot. You recorded the data in a Minitab worksheet and need to know whether you should accept or reject the entire lot of 2500 pipes. If the calculated Z value is greater than the critical distance (3.55750), you can accept the entire lot. See Example of creating a variables acceptance sampling plan.

1    Open the worksheet PIPESAMPLE.MTW.

2    Choose Stat > Quality Tools > Acceptance Sampling by Variables > Accept / Reject.

3    In Measurement data, enter 'Pipe thickness'.

4    In Critical distance (k value), enter 3.55750.

5    In Lower spec, enter 0.09.

6    In Historical standard deviation, enter 0.025.

7    Click OK.

Session window output

Interpreting the results

From the measurements of the 104 pipes that you sampled, the mean pipe thickness is 0.199739 inches and the historical standard deviation is 0.025 inches. The lower specification of the pipe thickness is 0.09 inches.

The critical distance was determined to be 3.5575 when you created the sampling plan originally. Because this is smaller than the calculated Z.LSL (4.38958), you will accept the lot of 2500 pipes.