In Control Chart Builder, the Weibull distribution is used to calculate the sigma value for a T chart. The estimates of the shape and scale parameters are calculated from the data and used to obtain the percentiles of the Weibull distribution.
Note: Subgroups with a response value of zero are given a weight of zero when estimating the Weibull distribution parameters.
Define the following quantities:
p1 = normalDist(-K) for Normal (0,1)
p2 = normalDist(0) for Normal (0,1)
p3 = normalDist(K) for Normal (0,1)
Then the limits are calculated using the following formulas:
LCL = Weibull Quantile (p1, β, α)
CL = Weibull Quantile (p2, β, α)
UCL = Weibull Quantile (p3, β, α)
where:
β is the shape parameter and α is the scale parameter for the Weibull Quantile function
K is the sigma multiplier and is set to 3 by default
For more information about the Weibull Quantile function, see Help > Scripting Index.