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Process capability indices

`S = capability(data,specs)`

`S = capability(data,specs)`

estimates capability
indices for measurements in `data`

given the specifications
in `specs`

. `data`

can be either
a vector or a matrix of measurements. If `data`

is
a matrix, indices are computed for the columns. `specs`

can
be either a two-element vector of the form `[L,U]`

containing
lower and upper specification limits, or (if `data`

is
a matrix) a two-row matrix with the same number of columns as `data`

.
If there is no lower bound, use `-Inf`

as the first
element of `specs`

. If there is no upper bound, use `Inf`

as
the second element of `specs`

.

The output `S`

is a structure with the following
fields:

`mu`

— Sample mean`sigma`

— Sample standard deviation`P`

— Estimated probability of being within limits`Pl`

— Estimated probability of being below`L`

`Pu`

— Estimated probability of being above`U`

`Cp`

—`(U-L)/(6*sigma)`

`Cpl`

—`(mu-L)./(3.*sigma)`

`Cpu`

—`(U-mu)./(3.*sigma)`

`Cpk`

—`min(Cpl,Cpu)`

Indices are computed under the assumption that data values are independent samples from a normal population with constant mean and variance.

Indices divide a “specification width” (between specification limits) by a “process width” (between control limits). Higher ratios indicate a process with fewer measurements outside of specification.

[1] Montgomery, D. *Introduction
to Statistical Quality Control*. Hoboken, NJ: John Wiley
& Sons, 1991, pp. 369–374.