Using Acceptance Sampling to Improve Quality

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Acceptance sampling may be applied where large quantizes of similar items or large batches of material being bought or are being bought or are being transferred from one part of organization to another. Unlike statistical process control where the purpose is to check production as it proceeds, acceptance sampling is applied to large batches of goods which have already been produced. Acceptance sampling is a method used to determine if you accept or reject a particular lot of products. This is done by first establishing an acceptance plan or sampling plan which sets the product acceptability criteria. This is known as the decision rule. The decision rule is an rule that explicitly states how many out-of-specification items (in a batch of arbitrary size) can be shipped to a customer. Depending on the type of sampling plan being utilized by the company, there could be more than one decision rule in effect (for double or multiple plans). Sampling plans utilizing can be single, double, or multiple. Single sampling plan consists of a sample size n and an acceptance number c. The lot is rejected if there is more than c defective for the sample size.

Acceptance Quality Control and Acceptance Sampling It was pointed out by Harold Dodge in 1969 that Acceptance Quality Control is not the same as Acceptance Sampling. The latter depends on specific sampling plans, which when implemented indicate the conditions for acceptance or rejection of the immediate lot that is being inspected. The former may be implemented in the form of an Acceptance Control Chart. Acceptance sampling is an important field of statistical quality control that was popularized by Dodge and Romig and originally applied by the U.S. military to the testing of bullets during World War II. If every bullet was tested in advance, no bullets would be left to ship. If, on the other hand, none were tested, malfunctions might occur in the field of battle, with potentially disastrous results. Basically the "acceptance quality control" system that was developed encompasses the concept of protecting the consumer from getting unacceptable defective product, and encouraging the producer in the use of process quality control by: varying the quantity and severity of acceptance inspections in direct relation to the importance of the characteristics inspected, and in the inverse relation to the goodness of the quality level as indication by those inspections." To reiterate the difference in these two approaches: acceptance sampling plans are one-shot deals, which essentially test short-run effects. Quality control is of the long-run variety, and is part of a well-designed system for lot acceptance. Acceptance sampling is a process of inferring the quality of a large number of items based on the quality of a small sample of the items. In acceptance sampling, we use the Operating Characteristic or OC curve to estimate the probabilty of making a Type I or Type II error. The OC curve plots the probability of accepting the lot (Y-axis) versus the lot fraction or percent defectives (X-axis). The OC curve is the primary tool for displaying and investigating the properties of an Acceptance Sampling acceptance sampling, by attributes each item tested is classified as conforming or non-conforming.(items used to classified as defective or non – defective but these days no self respecting manufacturing firm will admit to making defective items.) for this method to be effective, batches containing some non conforming items must be acceptable. Acceptance sampling can also be carried out by measuring variable rather than classifying an item as conforming or non conforming. Variables such as thickness, strengths or weight might be measured.

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