CuSum statistics

 

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Acceptance limit, starting value, sample unit tolerance, and C1/C2

The following statistics are calculated from values stored in the configuration and from values stored in the standard:

Upper Limit

The upper limit used for this equation depends on the Acceptance limit based on setting for this configuration.

The Upper individual limit, Upper gate, and Upper spec are stored in the standard.

Target

The Target is stored in the standard as Target X (Nominal).

A

A = Acceptance limit

A = Upper limit – Target

S

S = Starting value

S = k * A

where

k = Start value constant

T

T = Sample unit tolerance

T = ( k * A) / h

where

h = Decision interval value

k = Start value constant

C1 and C2

C1 = Target – T

C2 = Target + T

The C1 and C2 values are used to display the C1/C2 Zone on CuSum charts. They are also used to calculate the Upper and Lower CuSum values for each data subgroup.

Calculating and displaying CuSum values for each subgroup

The following statistics are calculated for each data subgroup entered.

QU(n)

QU(n) = Upper CuSum value for nth subgroup

QU(n) = QU(n-1) + XBar(n) - C2

where

QL(n)

QL(n) = Lower CuSum value for nth subgroup

QL(n) = QL(n-1) - XBar(n) + C1

where

Charting the top of the CuSum bar for a subgroup

Charting the bottom of the CuSum bar for a subgroup

Checking for CuSum failures in real time

During data entry, a data subgroup where QU(n) > A or QL(n) > A fails the real-time test for CuSum failures.