The Capability Analysis (Weibull) generates a process capability report containing various capability indices (Pp, Ppk, PPU, and PPL). It is used when the data is from a Weibull distribution. This procedure also generates a histogram of the data with a Weibull curve. ProcessMA uses maximum likelihood estimates for distribution parameters.
Interpret Capability Indices
Pp |
Pp measures the overall process capability, sometimes also referred to as long term performance. Therefore, if there are special cause variation present in the process data, Pp will be smaller than Cp. |
Ppk |
Ppk also measures the overall process capability but unlike Pp, it also take into consideration the location of the process mean in relation to the specification limits. Similar to the interpretation for Cp and Cpk, If the process is stable but off-target, Ppk will be smaller than Pp. |
Z |
Z is an alternative measure of process capability. In simple terms, Z represents the number of standard deviations between the mean and the specification limit. For 2-sided specification limits, overall Z is smaller than the Z of each specification limit. |
Observed Performance |
The number process output that actually fail the specification limits based on your data. This metric can be expressed in terms of parts per million (PPM) or in percent. |
Expected Performance |
The number of process output that you expect to fail specification limits. This level of performance is what you customer will likely experience. This expectation is based on the distribution (using cumulative distribution function) used to model your data. The expected performance can be expressed in terms of parts per million (PPM) or in percent. |
At the Excel Menu (For Excel 2007, go to Add-ins first)
Choose ProcessMA > Quality Tools > Capability Analysis > Capability Analysis (Weibull)
In Variable, select the column containing the data (Numeric)
In Lower Specification Limit, enter the lower specification limit (At least one limit is required, Numeric)
In Upper Specification Limit, enter the lower specification limit (At least one limit is required, Numeric)
In Historical Shape, enter the historical shape parameter value (Optional, Numeric, >0)
In Target, enter the target value. It is used to calculate Cpm (Optional, Numeric)
In Sigma Tolerance, enter the interval (number of standard deviations) to calculate the capability statistics, otherwise 6 standard deviations will be used (Numeric, >0)
In Display Capabilities in, choose the method to display capability indices
In Display Performance in, choose the method to display performance indices
Click OK
You are an engineer and you need to ensure the electrical stability of circuit boards. You recorded the length of time the circuit boards were unstable while being plugged in for a period of 30 days. You want to run a capability cnalysis based on the Weibull distribution.
Open data worksheet by choosing ProcessMA > Tools > Data
Choose ProcessMA > Quality Tools > Capability Analysis > Capability Analysis (Weibull)
In Variable, select AD - Unstable
In Upper Specification Limit, enter 3
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