Methods of Inspection
There are two methods of inspection. They are 100% inspection and Sampling inspection.
1. 100% Inspection
This type will involve careful inspection in detail of quality at each strategic point or stage of manufacture where the test involved is non-destructive and every piece is separately inspected. It requires more number of inspectors and hence it is a costly method. There is no sampling error. This is subjected to inspection error arising out of fatigue, negligence, difficulty of supervision etc. Hence complete accuracy of influence is seldomly attained.
It is suitable only when a small number of pieces are there or a very high degree of quality is required. Example : Jet engines, Aircraft, Medical and Scientific equipment.
2. Sampling Inspection
In this method randomly selected samples are inspected. Samples taken from different batches of products are representatives. If the sample prove defective. The entire concerned is to be rejected or recovered. Sampling inspection is cheaper and quicker. It requires less number of Inspectors. Its subjected to sampling errors but the magnitude of sampling error can be estimated. In the case of destructive test, random or sampling inspection is desirable. This type of inspection governs wide currency due to the introduction of automatic machines or equipments which are less susceptible to chance variable and hence require less inspection, suitable for inspection of products which have less precision importance and are less costly.
Example : Electrical bulbs, radio bulbs, washing machine etc. Destructive tests conducted for the products whose endurance or ultimate strength properties are required.
Example : Flexible strength, resistance capacity, compressibility etc.
Drawbacks of Inspection
(1) Inspection adds to the cost of the product but not for its value.
(2) It is partially subjective, often the inspector has to judge whether a product passes or not.
Example : Inspector discovering a slight burnish on a surface must decide whether it is bad enough to justify rejection even with micrometers a tight or loose fit change measurement by say 0.0006 inches. The inspectors design is important as he enforces quality standards.
(3) Fatigue and Monotony may affect any inspection judgement.
(4) Inspection merely separates good and bad items. It is no way to prevent the production of bad items.
