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Sigma level Long Term (Zlt) Sigma Level Short term (Zlt) A Z value is a data point's position between the mean and another location as measured by the number of standard deviations. For example, a 6 sigma process means that six standard deviations lie between the mean and the nearest specification limit. Six is the Z value. Zlt is calculated from data set which can have special causes or assignable causes Zst is calculated from a data free of any special cause or assignable cause Zlt is calculated for a longer duration of time in which there are some changes in the process. Zst is calculated in a shorter time frame when all the parameters remain the same We can measure Zsl and Zlt separately for a particular data set. But generally we measure one and compute the other with the formula Zst= Zlt+1.5. This formula is derived by Motorolla, by conducting many research from their historical data. For measurement purpose we can measure both separately, but when we want to check the improvements in a process. We should always measure Zst, because it gives correct reflection of our data. And estimate Zlt from Zst. In a six sigma project, we should always make Zst as our bench mark and make improvements on this.