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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.