Acceptance sampling
Introduction
When materials are purchased in a large quantity or articles are produced on a large scale, there may be defective items in each lot of materials, parts, components etc. purchased or finished products manufactured by complete inspection of the whole lot i.e. by 100% inspection acceptance and use of production of defective items can be avoided. But because of the following reasons, 100% inspection (piece to piece) is not done;
o The cost of inspection is high.
o Destructive methods of testing will result in 100% spoilage of the parts.
o Time taken for inspection will be too long and require more labour.
o When the population is large of infinite, it would be impossible or impracticable to inspect each unit.
Therefore, to assure better quality on one hand and to come out of the difficulties as mentioned above acceptance sampling method i.e. method sampling inspection is used.
Meaning of acceptance sampling
When the decision regarding acceptance or rejection of an entire lot of products or materials or parts or components is taken on the basis of number of defectives in a sample, it is called acceptance sampling.
Here, decision is taken without going through 100% inspection of the entire lot. Acceptance sampling reduces the work of inspection and the quality standards. The maximum limit of the number of defectives in a sample, to consider it acceptance, is determined first, and on the basis of limit, decision regarding acceptance or rejection is taken.
o If the number of defects in a sample is less than the prescribed number, the lot is accepted and if the number of defects is more than the prescribed number, the lot is rejected. Acceptance sampling is used to determine whether
o To accept or reject a particular lot of materials purchased or
o To pass or not to pass a particular batch of finished products for dispatch to the customers
o To pass or not pass a particular lot of materials in process.
Types of acceptance sampling
1. Attributes acceptance sampling
Here decision regarding acceptance or rejection is based on number of defectives found in a sample. A sample is taken from a lot and good items and defective items are separated. Then the number of defective items is less than the allowable limit, the lot is accepted, otherwise rejected. This type of sampling is used widely, because of its simplicity.
2. Variable acceptance sampling
Here decision regarding acceptance or rejection is based on ‘average’ (mean) and ‘spread’ of a number of individual measurements specifying the quality characteristics of a sample. Here a sample is taken, quality characteristics of each unit are measured, and average is worked out. The average value worked out is compared with allowable value stated in sampling plan, to decide whether to accept or reject a lot.
Type of sampling plan
Sampling plan may be three types. It may be;
1. Single sampling plan
If the number of defective pieces is greater than or equal to the rejection number (in case of a single sampling plan, rejection number is equal to the acceptance number plus one), then the lot is rejected.
Example
Lot size (N) = 5000
Sample size (n) = 150
Acceptance number (c) = 2
The following data suggests for selecting 150 pieces at random from the lot of 5000 pieces and inspecting them for the quality standard. If the number of defective is 0 to 2, accept the lot and if the number of defective is 3 (rejection number r= c+1) or more, reject the lot.
2. Double sampling plan
It is a sampling inspection in which the inspection results of two samples are used to decide whether to accept or reject the complete lot.
Example
Lot size (N) = 5000
Sample size (n1) = 150 and n2 =50
Acceptance number (c1) = 0 and c2 = 3
Rejection number r1 =3 and r2 = 4
The above data suggests selecting 150 pieces at random from the lot of 5000 pieces first and then inspecting then from the quality standard. If the number of defectives is 0 (c1) or less, accept the lot and if the number of defective is 3(r1) or more, reject the lot.
But if there is only 1 or 2 defectives (between c1 and r1), select the second sample of 50 pieces for inspection against quality standard. If the number of defectives is the combined sample (n1+n2) is 0 to 3, accept he lot and if the number of defectives in the combined samples is 4 (r2) or more, reject the lot.
3. Multiple (continuous) sampling plans.
Under this plan, several are taken until a decision to accept or rejected lot is reached. It is known as sequential sampling plan also. Is should be noted that if the first new items are found defective, the lot could be rejected without further inspection.
Which ever may be sampling plan, selection of a sample is made at random, so that all the items in a lot have an equal chance of selection in a sample. Acceptance sampling plan specifies the sample size (number of items in a sample) for a specific lot size and the number of allowable defects in the sample, on the basis of which decisions regarding acceptance or rejection of the entire batch or lot is made.
Introduction
When materials are purchased in a large quantity or articles are produced on a large scale, there may be defective items in each lot of materials, parts, components etc. purchased or finished products manufactured by complete inspection of the whole lot i.e. by 100% inspection acceptance and use of production of defective items can be avoided. But because of the following reasons, 100% inspection (piece to piece) is not done;
o The cost of inspection is high.
o Destructive methods of testing will result in 100% spoilage of the parts.
o Time taken for inspection will be too long and require more labour.
o When the population is large of infinite, it would be impossible or impracticable to inspect each unit.
Therefore, to assure better quality on one hand and to come out of the difficulties as mentioned above acceptance sampling method i.e. method sampling inspection is used.
Meaning of acceptance sampling
When the decision regarding acceptance or rejection of an entire lot of products or materials or parts or components is taken on the basis of number of defectives in a sample, it is called acceptance sampling.
Here, decision is taken without going through 100% inspection of the entire lot. Acceptance sampling reduces the work of inspection and the quality standards. The maximum limit of the number of defectives in a sample, to consider it acceptance, is determined first, and on the basis of limit, decision regarding acceptance or rejection is taken.
o If the number of defects in a sample is less than the prescribed number, the lot is accepted and if the number of defects is more than the prescribed number, the lot is rejected. Acceptance sampling is used to determine whether
o To accept or reject a particular lot of materials purchased or
o To pass or not to pass a particular batch of finished products for dispatch to the customers
o To pass or not pass a particular lot of materials in process.
Types of acceptance sampling
1. Attributes acceptance sampling
Here decision regarding acceptance or rejection is based on number of defectives found in a sample. A sample is taken from a lot and good items and defective items are separated. Then the number of defective items is less than the allowable limit, the lot is accepted, otherwise rejected. This type of sampling is used widely, because of its simplicity.
2. Variable acceptance sampling
Here decision regarding acceptance or rejection is based on ‘average’ (mean) and ‘spread’ of a number of individual measurements specifying the quality characteristics of a sample. Here a sample is taken, quality characteristics of each unit are measured, and average is worked out. The average value worked out is compared with allowable value stated in sampling plan, to decide whether to accept or reject a lot.
Type of sampling plan
Sampling plan may be three types. It may be;
1. Single sampling plan
If the number of defective pieces is greater than or equal to the rejection number (in case of a single sampling plan, rejection number is equal to the acceptance number plus one), then the lot is rejected.
Example
Lot size (N) = 5000
Sample size (n) = 150
Acceptance number (c) = 2
The following data suggests for selecting 150 pieces at random from the lot of 5000 pieces and inspecting them for the quality standard. If the number of defective is 0 to 2, accept the lot and if the number of defective is 3 (rejection number r= c+1) or more, reject the lot.
2. Double sampling plan
It is a sampling inspection in which the inspection results of two samples are used to decide whether to accept or reject the complete lot.
Example
Lot size (N) = 5000
Sample size (n1) = 150 and n2 =50
Acceptance number (c1) = 0 and c2 = 3
Rejection number r1 =3 and r2 = 4
The above data suggests selecting 150 pieces at random from the lot of 5000 pieces first and then inspecting then from the quality standard. If the number of defectives is 0 (c1) or less, accept the lot and if the number of defective is 3(r1) or more, reject the lot.
But if there is only 1 or 2 defectives (between c1 and r1), select the second sample of 50 pieces for inspection against quality standard. If the number of defectives is the combined sample (n1+n2) is 0 to 3, accept he lot and if the number of defectives in the combined samples is 4 (r2) or more, reject the lot.
3. Multiple (continuous) sampling plans.
Under this plan, several are taken until a decision to accept or rejected lot is reached. It is known as sequential sampling plan also. Is should be noted that if the first new items are found defective, the lot could be rejected without further inspection.
Which ever may be sampling plan, selection of a sample is made at random, so that all the items in a lot have an equal chance of selection in a sample. Acceptance sampling plan specifies the sample size (number of items in a sample) for a specific lot size and the number of allowable defects in the sample, on the basis of which decisions regarding acceptance or rejection of the entire batch or lot is made.
Comments
Post a Comment