Though variable control charts i.e., x̄ – and R -charts are powerful tools for the diagnosis of quality problems, they do have certain limitations. The limitations of x̄ – and R-charts are as follows:
- x̄ and R-charts cannot be used for quality characteristics that are attributes. For example, quality characteristics such as incorrect colour, blow holes, cracks are not measurable; and a variable control chart is not applicable.
- x̄ and R-charts can be used only for one measurable characteristic at a time. For example, an automatic turning may have a 50 dimensions and therefore 50 x̄-and R-charts would be required. Thus this method is too expensive and time consuming.
In order to avoid the above limitations, control charts for attributes are used.
Attributes
✔ What is meant by attribute?
An attribute refers to those quality characteristics that conform to specifications or do not conform to specifications.
✔ Two types of attributes are:
(i) Attributes where measurement are not possible. For example, visually suspected items such as colour, missing parts, scratches and damage.
(ii) Attributes where measurements can be made but are not made because of economy of time, cost or need. In common practice, instead of using measuring instruments such as micrometer, vernier caliper, etc. we use ‘Go – No Go Gauges’ to conform it good or bad.
✔ Non-conformity Vs. Non-conforming Item (Defect Vs. Defective):
- An item is said to be a non-conforming item or defective if it fails to conform to the specifications in any of the characteristics.
- Each characteristics that does not meet the specifications is a non-conformity or defect.
- A non-conformity refers to a quality characteristic. Since an item may have many quality characteristics (say, length and diameter of a shaft), it may have many non-conformities. Non-conforming refers to the entire item. Thus a non-conforming item may have many non-conformities.
- Examples:(i) If a casting contains undesirable hard spots, blow holes, the casting is non-conforming item (defective) and the hard spots, blow holes, etc., are the non-conformities (defects).
(ii) A paint bubble on the surface of a newly manufactured car may be a non-conformity, yet the car was shipped as conforming.
