Definition - Inspection, Test and Measurement in Quality
Inspection can be defined as the formal evaluation exercise which studies the characteristics of the product to ensure conformance. They can be called as reviews, product reviews, audits and walk through. By doing so, it gives an assurance that the product will be built in accordance with project specifications, code and standard requirements, and meet local government regulations. Inspection thus is a technique for quality control. Inspections are generally conducted at different levels - there might be inspection at one single level or it could be inspection of the final product of the project.
The process of raw material inspection is known as receiving inspection. The organisation which receives raw material has to make sure that the product is not processed or used, until the completion and verification of the product is over. The organisation performs the verification to check conformity with the quality plan and documented procedures and if warranted, do the initial inspection at the subcontractor's premise.
During inspection, you must verify the suitability of the product against measurements and the exactness required to express the conformance of the product to the specifications.
The inspection and the test plans should identify the product which is to be tested and inspected. It should clearly define the requirements and criteria for acceptance. The inspection procedures at every stage are to be defined. The parameters used for conformance must include those which are to be verified by the manufacturing procedures.
Testing of products is a very important and mandatory step. The procedures for test planning and inspection include:
- Preparing the requirements that describe the product and the characteristics that have to be verified.
- Produce the forms to record the results of tests and inspection.
Test procedures describe the tasks, techniques and operational steps and control that are meant for specific method of testing and test objects.
- To determine conformity inspection and test typically include measurement of an output and comparison to specified requirements. Inspection is performed for a broad variety of purposes, e.g. distinguishing between good and bad product, determining if a process is changing, measuring process and capability, rating product quality, securing product design information, accuracy and determining the precision of measuring instruments. The nature of inspection and the manner of doing it has its special influence on each of the above mentioned.
- The difference between "inspection" and "test" has become blurred.
Inspection, typically performed under static situations on items such as components, can vary from simple visual examination to a series of complex measurements. The emphasis on inspection is more because it helps to determine conformance to a standard. Test, on the other side, is performed under complex items such as subassemblies or systems. Conformance is determined from the test results and can also be used as input for other analyses such as evaluating a fresh design diagnosing problems, or making physical adjustments on products.
- The inspection planning depends mainly on 3 aspects:
- The completeness of inspection planning.
- The bias (efficiency) and precision of the instruments.
- The human error level.
- High error rates are particularly prevalent in inspection tasks having a great degree of monotony, e.g.: jars of a food product for foreign particles being viewed, luggage's being screened at an airport security gate. Surprisingly, monotony that causes an inspector to miss defects can be built in a short duration. With monotonous inspection, inspectors detect about 80 to 90 % of the defects and miss the remainders. Hence,
100% inspection that is monotonous is not truly 100% effective in detecting defects. One of the advantages of automated inspection is the elimination of human error.
- In the case of human error in inspection, this arises from multiple causes, of which four are most vital:
- Technique errors.
- Inadvertent errors.
- Conscious errors.
- Communication error.
The characteristics or features of a measurement system are described in five different ways:
Stability: This refers to the capability of a measurement system to create the same values over time when calculating the same sample. In case of statistical process control charts, constancy means the lack of special cause variation.
Measurement bias: This refers to the systematic error caused due to the measurement instrument. Sometimes the measurement process is considered biased if it periodically overstates (gives higher value) or understates the actual value.
Linearity: This is a calculation of the constancy of Bias over the range of the measurement equipment. For example, if a bathroom scale is less than 1.0 pound when calculating a 150 pound person, but is off by 5.0 pounds while calculating a 200 pound person, the scale Bias is non-linear and the degree of Bias varies over the range of use.
Repeatability: This gauge whether the same appraiser can calculate the same sample many times with the same measurement apparatus and obtain the same value.
Reproducibility: This calculates whether diverse appraisers can calculate the same part/sample with the same measurement device and get the same value.