Table of Contents:-
- What is acceptance sampling?
- What is an acceptance sampling plan?
- Advantages of acceptance sampling
- Limitations of Acceptance Sampling
- Types of Acceptance Sampling Plans
What is acceptance sampling?
Acceptance sampling is a sampling inspection in which a decision about the acceptance or rejection of a lot is based on one or more samples that have been inspected.
Acceptance sampling is a technique in which a small part or fraction of the units/items is randomly selected from a lot, and these chosen units are inspected to decide whether the lot should be accepted or rejected based on the information provided by the sample inspection.
For example, let’s consider a manufacturer of cricket balls who supplies them in lots of 500. A buyer wishes to inspect 20 balls from each lot before deciding to accept or reject the entire lot, relying on the information provided by this sample. This scenario exemplifies acceptance sampling as the decision regarding the lot is made based on a sample.
In another example, a mobile phone company packs the mobile phones it produces in lots of 100 units. To check the quality of the lots, the company’s quality inspector draws a random sample of size ten from each lot. They then decide whether to accept or reject the lot based on the information provided by this sample.
Quality Control and Rejected Lots in Acceptance Sampling
When a lot is rejected through acceptance sampling, it does not mean all units are unusable or defective. Instead, it means that all units in the lot have been carefully examined, and any faulty units have either been repaired or removed. Therefore, the remaining units in the lot conform to a certain quality level. The rejected units may be sold at a lower price, or the defects in the units may be rectified to retain them. For instance, clearance sales, such as those for ready-made clothes or shoes, are carried out to sell items that need to meet specifications or have been rejected. This results in a loss for the producer or manufacturer, which could be prevented if they could maintain the desired level of quality. Therefore, manufacturers should be cautious about the quality of goods they supply to customers.
What is an acceptance sampling plan?
An acceptance sampling plan is a specific plan that clearly outlines the rules for sampling and the criteria for accepting or rejecting a lot. Acceptance sampling plans can be employed for the inspection of the following points:
1. Manufactured units/items,
2. Components,
3. Raw materials,
4. Operations,
5. Materials in process,
6. Supplies in storage,
7. Maintenance operations,
8. Data or records, and
9. Administrative procedures.
Advantages of acceptance sampling
The main advantages of acceptance sampling are as follows:
i) It costs less money, time, and labour than 100% inspection.
ii) For items that cannot be used after a single inspection, such as crackers, bulbs, tube lights, food, etc., more than 100% inspection is needed. Sampling inspection is the only viable method for inspecting such items.
iii) Acceptance sampling requires the inspection of a small sample of items or units, leading to a more minor inspection staff requirement.
iv) In many cases, acceptance sampling provides better outgoing quality. Generally, it is acknowledged that even with a good 100% inspection, only 85% to 90% of defective items are removed, while an excellent 100% inspection removes only 99% of faulty items. However, it typically does not reach the 100% mark due to human error. In other words, 100% inspection is only sometimes reliable because it involves too much routine work for those inspecting each item. Consequently, defective items may be incorrectly labelled as satisfactory and accepted when the inspectors are distracted. Hence, an appropriate sampling plan is preferable.
v) Due to quick inspection through acceptance sampling, scheduling and delivery times are saved.
Limitations of Acceptance Sampling
Acceptance sampling has some limitations, which are given below:
i) In acceptance sampling, the entire lot is accepted or rejected based on conclusions drawn from one or more samples, leading to some risk of making incorrect inferences about the quality of the lot.
ii) The success of acceptance sampling depends on the randomness of the sample, quality characteristics to be tested, lot size, acceptance criteria, etc. Therefore, it is a specialized job that requires careful planning and execution, and not everyone can undertake it.
So far, you have learned about the acceptance sampling plan and its advantages and limitations. It is also essential for you to know about the different types of acceptance sampling plans used in industries.
Types of Acceptance Sampling Plans
Several types of acceptance sampling plans based on different approaches can influence the decision about a lot. These are categorized as follows:
1. Acceptance sampling plans for attributes, and
2. Acceptance sampling plans for variables.
1. Acceptance Sampling Plans for Attributes
Acceptance sampling plans are known as acceptance sampling plans for attributes, where actual measurements of the quality characteristics are not made. Still, the units/items are categorized as defective or non-defective based on Go-No-Go gauges. These plans are divided into two types:
A. Lot-By-Lot Acceptance Sampling Plans for Attributes
Lot-by-lot acceptance sampling plans for attributes are the most commonly used and are referred to as acceptance sampling plans for attributes. They are employed when the units to be inspected can be conveniently grouped into batches or lots. In lot-by-lot acceptance sampling plans for attributes, we generally use the following plans:
i) Single Sampling Plans: The decision about accepting or rejecting a lot is based on a single sample that has been inspected. This is the simplest type of sampling plan.
ii) Double Sampling Plans: The decision about accepting or rejecting a lot requires evidence from two samples drawn from the lot. If the lot quality is good (or bad), the lot is received (or rejected) based on the first sample. If the first sample shows intermediate quality, the decision about the lot is made using evidence from both the first and second samples. This is more complex than the single sampling plan.
iii) Multiple Sampling Plans: An extension of the double sampling plan, this plan may require more than two samples to decide about the acceptance or rejection of a lot.
iv) Sequential Sampling Plans: In the sequential sampling plan, the sample size is not pre-fixed, as in single, double, and multiple sampling plans. Units/items are drawn from the lot one at a time and inspected. The decision about accepting or rejecting the lot or continuing with the inspection by taking one more unit from the lot is made based on the available information up to that stage. The multiple and sequential sampling plans are beyond the scope of this course.
B. Continuous Production Acceptance Sampling Plans for Attributes
Continuous production acceptance sampling plans for attributes are used when the units/items to be inspected cannot be grouped into lots or batches. Many manufacturing operations do not create lots because, in these operations, units are produced in a continuous process on a conveyor belt or other straight-line systems. Ongoing production acceptance sampling plans for attributes are beyond the scope of this course.
2. Acceptance Sampling Plans for Variables
Acceptance sampling plans for variables involve actual measurements of the quality characteristics. However, these plans are beyond the scope of this course. Therefore, we now focus on acceptance sampling plans for attributes and describe the general implementation procedure.