Principle Of Acceptance Sampling

Introduction

Acceptance sampling by Attributes is the most common type of sampling. A predetermined number of units (sample) from each lot is inspected by attributes. If the amount of defectives is less than or equal to the prescribed minimum the lot is accepted, if not the lot is rejected as it falls below standard.

A single sampling plan is determined by a lot size (N), the sample size (n) and the acceptance number (c). For example consider a sampling plan having N = 9000, n = 300 and c = 7. This means that a lot of 9000 units has 300 units inspected and if 7 or less defectives are found in 300 units of sample, the lot is accepted. If more than 7 defectives are found in the sample of 300 units the lot is rejected.

Acceptance sampling can be performed in a number of different situations where customer-producer relationships exist. The customer and producer can be two different companies, two plants within the same company or two departments within the same plant.

In any case there is always the problem associated with the acceptance of goods. Acceptance sampling of the product is most likely to be used in one of the following three situations :

1. When the test is destructive, sampling plan is necessary otherwise all the products will be destroyed by testing.

2. When the cost of 100% inspection is very high, sampling can save money.

3. When there are many similar items to be inspected, the sampling inspection will produce as good, if not better results than 100% inspection. This is true because the manual or 100% inspection causes fatigue and boredom and results in a very high rate of defective material.

Advantages and Disadvantages of the sampling plan compared with 100% inspection

Advantages :

1. More economical owing to fewer inspection.

2. Less handling damages during inspection.

3. Upgrading the inspection job from piece by piece decision to lot by lot.

Disadvantages :

1. There is a certain risk of accepting the bad lot and rejecting the good lot.

2. More time and effort is devoted to plan the documentation.

3. Less information is usually provided about the product.

Comparison between 100% inspection and sampling inspection.

Guidelines for the Formation of Lots

1. The lot should be homogenous that is all the products in the lot are produced by the same machine, same operator and same input material.

2. The lot should be as large as possible.

3. Lots should be suitable for material handling.

4. Lots should be confirmed to packing principle.

TYPES OF SAMPLING PLANS

1. Single sampling plan.

2. Double sampling plan.

3. Multiple sampling plan.

4. Sequential sampling plan.