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The long-term z-score, Zlt, is a long-term process capability metric. It is the distance in Standard Deviations (sigma) between the long-term process mean and the closest specification limit. It is calculated by subtracting 1.5 (the widely assumed long term process drift) from the Zst, or by using long-term data in the function: (Spec limit - Long Term Mean)/Long Term Std. Dev. Long-term data is obtained by taking Samples which take into account sources of Common Cause variation as well as Special Cause variation. Zlt can be used to estimate DPMO.

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