Publication date: 04/12/2021

Control Limits for T Charts

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)


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.

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