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Sampling Plan Analyzer Help System 

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Double Sampling Plan for Defective Units

Characteristic:

Defective units

Assumptions:

(1) Representative sample

Parameters:

n1 = first sample size, integer satisfying n12
a1 = first accept number, integer satisfying a1
0 and a1n1-2
r1 = first reject number, integer satisfying r1
a1+2 and r1n1
n2 = second sample size, integer satisfying n2>r1-a1-2
a2 = second accept number, integer satisfying a2
r1-1 and a2<n2+a1+1

Sometimes a sixth parameter, r2 = second reject number, is given. The parameter r2 is always equal to a2+1 and is redundant.

Procedure:

(1) Take a
representative sample of n1 units from the lot to be inspected.
(2) Count the number of defective units in this sample.
(3) If this count is less than or equal to a1, accept the lot.
(4) If this count is greater than or equal to r1, reject the lot.
(5) Otherwise, take a second representative sample of n2 units from the lot.
(6) Count the total number of defective units in the combined sample of n1+n2 units.
(7) If this count is less than or equal to a2, accept the lot. Otherwise, reject the lot.

Sometimes the accept or reject decision is determined before all units are inspected. In this case, the inspection can be
curtailed. For double sampling plans for defective units you can curtail on both acceptance and rejection during both stages as follows:

Curtail on Rejection during First Stage: Stop the inspection and reject as soon as r1 defective units are found.
Curtail on Acceptance during First Stage: Stop the inspection and accept as soon as n1-a1 good units are found.
Curtail on Rejection during Second Stage: Stop the inspection and reject as soon as a2+1 defective units are found.
Curtail on Acceptance during Second Stage: Stop the inspection and accept as soon as n1+n2-a2 good units are found.