According to common man's usage control means "making something to behave in a way we want it to behave". But the technical meaning of control is different. In fact control is standard of comparison for VERIFYING or CHECKING the findings of a process.
But upon extending the concept of control to quality control, one finds that above definitions lack in one vital issue - FEEDBACK. Thus, a reined version would be as quoted by Juron as, "The process of measuring quality performance comparing it to the requirements and acting on the difference".
Thus, quality control is a regulatory process through which we measure actual performance, compare it with the standards set for it and act on the difference.
The purpose of quality control activities does not restrict to manage the quality but even to maintain the assurance of continued high quality of products and service. Like any other control activity it also involves the measurement, feedback, comparison with the standards and correction where necessary. The word measurement immediately tells us that there is one more aspect of quality characteristic, i.e. qualitative aspect.
Again qualitative aspect of quality itself leads to following two different categories :
- Attributes, and
- Variables.
Attributes are actually countable. Situations like present or not present (a dichotomous classification). For example, materials may be defective or non-defective, accepted or rejected, go or no go.
For attributes data, discrete distributions such as Binomial and Poisson are used to make inferences about the population characteristic being controlled.
Variables are actually measurable. Examples of measurable quantities are dimensions. weights, temperature and times. For variable data, we use continuous distributions such as normal distribution.
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