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Minitab determines the mean from your data. The mean is the average of the data, which is the sum of all the observations divided by the number of observations.
The historical standard deviation is the known standard deviation of your process. Use a historical standard deviation when you have collected enough data over time to state with confidence what the process standard deviation is. If the process is stable and in control, then you can use a historical standard deviation instead of a calculated standard deviation.
When you do not specify a historical standard deviation, Minitab calculates the standard deviation from the data. If you have 2 specification limits and do not specify a historical standard deviation, then you use the standard deviation in the decision about whether to accept or reject the lot. If the Z-values are greater than the critical distance, and if the standard deviation is less than the maximum standard deviation, then accept the entire lot. Otherwise, reject it.
The lower specification limit (LSL) is the minimum allowed value for the product or service. This limit does not indicate how the process is performing but how you want it to perform.
The upper specification limit (USL) is the maximum allowed value for the product or service. This limit does not indicate how the process is performing but how you want it to perform.
You must specify at least one specification limit for a variables acceptance sampling plan.
Use the LSL and USL to define customer requirements and to evaluate whether your process produces items that meet the requirements.
Minitab compares your process data to the specification limits to determine whether to accept or reject an entire lot of product.
If the Z-values are greater than the critical distance, and if the standard deviation is less than the maximum standard deviation, then accept the entire lot. Otherwise, reject it.
The critical distance is the value that Minitab uses to compare with the sample mean and specification limits to determine whether to accept or reject a lot.
For example, suppose you sample lots of plastic pipes. Your sample plan calls for randomly sampling 104 of the 2500 pipes in a shipment. The lower specification for wall thickness is 0.09 inches. Minitab determines the critical distance to be 3.55750.
Minitab calculates the maximum standard deviation (MSD) when you provide both LSL and USL and do not provide a historical standard deviation.
If the Z-values are greater than the critical distance, and if the standard deviation is less than the maximum standard deviation, then accept the entire lot. Otherwise, reject it.
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