Any inspection procedure involving 100% inspection needs huge expenditure of time, money and labour. Expenditure on inspection is always considered as ‘dead weight cost.’ Moreover, due to boredom and fatigue involved in the repetitive inspection process there is always a possibility to overlook some defective item even by most competent and efficient inspectors.
Also, the nature of items may be such that these may be completely destroyed during the Process of inspection e.g. life of a candle. Evidently in such cases 100% inspection cannot be planned.
The alternative is statistical sampling inspection methods. Here, from the whole lot some items are selected for inspection and a decision regarding the quality of the whole lot is taken on the basis of the sampled items.
Image Source: itworks-inc.com
ADVERTISEMENTS:
The items can be selected by various sampling methods. Here, the whole lot is accepted if the sample items conform to the specifications otherwise it is rejected. Thus, the sample items are considered to be the representatives of the whole lot. The method of rejecting or accepting a lot on the basis of a sample is known as Acceptance sampling or sampling inspection.
Sampling inspection involves the checking of sample(s) from lots or batches. A formal name for sampling inspection is Acceptance sampling. Acceptance sampling is done by attributes and by variables. Acceptance sampling by attributes involves a decision such as go or no go, good or bad, yes or no, defective or not defective.
Examples of this include the checking of meat, vegetables, fruit, furniture finish, photographs, hardware items and stationery. Acceptance sampling by variables requires actual measurements. For example, the length, thickness, and diameter of a piston pin; the surface finish and hardness of a crank shaft, the module of a gear.
ADVERTISEMENTS:
Acceptance sampling, both by attributes and by variables, can be applicable for a single item. For example, the length of a metallic bolt fastener may be checked by a venire calipers, this becomes inspection by variables.
The threads on the bolt may be checked by a thread ring gauge for the pitch, diameter, and roughness of the threads. This is inspection by attributes. If the go side of the ring gauge accepts the bolt and the no-go side rejects it, the threads will be considered correct to the specifications. If not, the bolt will be rejected.
Procedure of Acceptance Sampling:
Process of acceptance sampling involves the following steps (Fig. 20.2)
Advantages of Acceptance Sampling/Sampling inspection:
The advantages of sampling inspection are as follows:
ADVERTISEMENTS:
(i) Items of destructive nature during inspection can be inspected by sampling only.
(ii) Economy of time and money in comparison to 100% inspection.
(iii) Problem of inspection fatigue occurring in 100% inspection is eliminated.
(iv) Small inspection staff required.
(v) Due to quick inspection, scheduling and delivery times are improved.
(vi) Can exert more effective pressure on quality improvement than the rejection of individual items.
Limitations of Acceptance Sampling:
Limitations & draw backs of acceptance sampling are:
Since the conclusion is based on a sample, there is always some likelihood or risk of making wrong inference about the quality of the lot. This is termed as producer’s risk or consumer’s risk.
The success of the scheme depends on randomness of samples, quality characteristics to be tested, lot size, acceptance criteria etc.