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Capability indices use estimated sigma.
Cp for one-sided specifications
If you are using one sided specifications, use the following formulas to determine the Cp:
Upper specification
Lower specification
Where:
Zmin is the smaller of Zupper and Zlower.
Using estimated sigma:
T = specification target (nominal)
Xi = a given individual reading of ” i ”
n = total number of individual readings
= symbol for summation
The t-chart formula:
The g-chart formula:
The c-chart formula (for number of nonconformities, from subgroups of a constant size):
The u-chart formula (for number of nonconformities from subgroups that can vary in size):
The p formula (for the proportion of nonconforming units from subgroups that can vary in size):
To calculate control limits for the p-chart:
The np-chart formula (for number of nonconforming units from subgroups with a constant size):
= the mean of the X-bars T = specification target Sigma is an estimated sigma or sigma of the individual values, depending on how the histogram is configured or whether you are calculating Cpk (estimated) or Ppk (actual).