Although it is not my first post about inspection, I still feel the message on controlling defects using inspections as a mean need to be detailed further.
In conventional production management, the defective products are identified and removed from the process through inspection methods. The inspection methods are either manual or using some automated technologies. Usually, an inspection process is verifying the characteristics against the standards to decide the acceptability of a product.
In order to understand, why the inspection process is not reducing the defects, we need to go deep down into the inspection process. The following are the basics of any inspections;
Inspection is done only after completion of a product or a work-in-progress product which is partially completed. By the time you inspect a product, the defect might have already occurred. Hence the inspection process only counts the defective products and separate from the lots from dispatch to the next stage but not reducing the defects.
If a sampling methodology is used for inspection, then this further worsens the situation as there is a huge opportunity of defective items being missed out from inspections. Though the sampling is statistically controlled, it neither assures the reduction in defects nor reducing the defects. It only emphasizes the no. of defective products in an entire lot.
Also, the inspection process is depending on inspector’s skill, his experience, his state of mind and physical condition, measuring instruments capability, calibration, environment in which the measurement is taken, standards used for reference and interpretation of these standards If you accumulate the error percentage in each of the dependents the resulting error would be higher.
While the measurement system analysis focuses only on instrument error, operator error on repeatability and reproducibility and the operational procedure mainly focuses on inspection methodologies, many errors are not being considered in the inspection processes (sure, I can write a separate post on this!). So, by reducing the error margin in the measurement system and procedures the inspection process becomes more accurate and rigid, however, the product quality is still not assured.
Shigeo Shingo says that there are two enhancing methods normally applied in the conventional inspection processes. That is about increasing the number of inspectors whenever more defects are occurring or conducting more rigorous inspections. However, he agrees, both of these enhancements are further increasing the problem. These methods are anyhow, not helping us to reduce the defects. He suggests conducting source inspections to reduce the defects (will talk about this in a separate post!).
Merely, inspections to be carried out when a regulatory or customer requirements anticipating the confirmation of quality for their confirmation and contractually beneficial to the organization. Otherwise, carrying out inspections must be looked at a non-value adding activity and should be avoided by ensuring the process quality. The approach for reducing defects must be on the processes than on inspections. The focus should be on eliminating the defects rather than sorting out the defectives.