How much inspection?

Excerpted from Ishikawa, Kaoru, "Introduction to Quality Control", 3A Corporation, Tokyo, 1994, pp.379-380.

  1. When is 100% inspection required?

    1. 100% inspection is needed when passing even a single defective will cause trouble, e.g., when a defect is critical and may endanger human life, or when shipping even one defective product will have a great impact either economically or from the viewpoint of credibility. One should note, however, that there is little point in carrying out 100% inspection designed to eliminate defectives completely if the product is liable to become defective during transport.
    2. When the product is extremely valuable, e.g., complete aircraft, ships, cranes, automobiles, etc.
    3. When the product's performance cannot be tested until it is assembled: in this case, 100% inspection should be carried out after assembly.
    4. When lots prepared for delivery include defective items, pre-delivery inspection should in principle be 100% inspection.

  2. When is 100% inspection advantageous?

    1. When 100% inspection can be carried out easily and reliably (e.g., checking whether light bulbs work) or when reliable automatic screening equipment can be used.
    2. When the lot size is small and there is no economic advantage in carrying out sampling inspection.
    3. When the lot average percent defective is far greater than the percent defective required.

  3. When is sampling inspection advantageous?

    1. When 100% inspection is imperfect.
    2. When there are many characteristics to be inspected.
    3. When inspection costs are high.
    4. When it is desired to guarantee the quality of product lotsat a specified percent defective.
    5. When it is desired to motivate a producer to raise quality and practice quality control.
    6. For receiving inspections.

    Purchased products should in principle either be subjected to sampling inspection or accepted without any inspection, and 100% inspection should be carried out by the supplier.

  4. When is sampling inspection required?

    1. For destructive inspection.
    2. For chemical analyses.
    3. When 100% inspection is impossible because of the sheer volume of products passing through.
    4. Whe the purchaser cannot rely on the supplier's quality assurance.

  5. When is zero inspection acceptable?

    Zero inspection is only acceptable when the process is stable and in the controlled state, and:

    1. When it can be concluded that every characteristic of the product amply satisfies the specifications and that all products are good, e.g., when the process is in the controlled state at a process capability index Cp of 1.67 or above.
    2. When the lot percent defective is clearly better than the percent defective required. However, even when this is so, it is better to perform occasional inspections as a check.