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Switching Rules

Switching Rules vs Sequential Sampling

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INTRODUCTION -- This page will compare two strategies that vary the sample size depending on lot quality:

  1. Switching rules from ASQ/ANSI Z1.4 and Z1.9 -- normal, tightened, and reduced inspection.
  2. Sequential sampling plans.

I prefer sequential sampling plans (2) over the switching rules (1) for the following reason.

SWITCHING RULES -- The switching rules switch back and forth between three sampling plans that have different oc curves. That is, the normal plan, the tightened plan, and and the reduced plan each has its own oc curve. For example switching from normal to reduced inspection gives you a plan with a worse consumers point. It is like switching from oc curve B to oc curve A below:

img00002.gif (2338 bytes)

Strategically, reduced inspection is like saying:

"Quality has been good recently so lets reduce the sample size and increase our exposure to passing off-grade lots."

When an off-grade lot finally comes along the reduced inspection plan has higher probability of accepting it -- compared to the normal plan.

SEQUENTIAL SAMPLING PLANS -- Sequential sampling plans have a constant oc curve. Unlike the switching rules, both the consumers point and the producers point of sequential plans are the same for every lot.

img00001.gif (2465 bytes)

Sequential sampling plans determine the sample size using only data from the CURRENT lot. The ASN curve of a sequential plan evaluates the effect of lot quality on the sample size.

To summarize, I think that sequential sampling plans are strategically better than switching rules.

IMPLEMENTING SEQUENTIAL SAMPLING PLANS -- Acceptance sampling software can develop sequential sampling plans.

DISCUSSION FORUM -- You can collaborate with others to get additional questions discussed at the sampling plan forum .
Author --  Stan Hilliard, CQE,CRE,CQA,PE


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