Variable Sampling

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Definition: Variable Sampling

Variable sampling is used for measurement of hardness , tensile strength , resistance and  other physical  properties of the desired sample. The variables used for measurement are on a continuous scale and the limits for rejection are clearly demarcated for the entire range. The sample which is tested from the complete lot is said to be its representative. 

It is a much more descriptive of the type of defect and the gives the measure of deviation from the accepted standard. The reason for declaring the product to be defective can easily be inferred from the result . It is preferred to an attribute sampling where the product is categorized as ok or defective . The attribute sampling does not indicate the exact reason for failure of a sample. It usually takes a large sample size so that there is  at-least a  couple of defective pieces  and  the failure rate can be expressed as a percentage. It  is considered to be a wasteful activity  because after  100% testing  we end up classifying them as tested ok or faulty.

Example :


The  sample size  selected  for testing  is between  4-25 . The sample size is kept low  because a homogenous quality is ensured during designing and  unnecessary expenditure is not done on testing. All the indicative parameters have to be recorded for the selected samples which makes it all the more time consuming. In the sample set for variable sampling the original values are taken as reference values and   the measured values are compared with them and the acceptance limits is decided by the  confidence  interval  restricted  by the normal distribution curve  and all the values lying outside them are  rejected . The curve is divided into three zones  .Zone 1 and 3 are  short zones and  Zone 2 is known as long zone .  The upper limits and lower limits of deviation  are  kept as +3 sigma and -3 sigma respectively . Hence it not only saves a lot of inspection cost it also keeps a check on controlling all the risk levels .

Hence, this concludes the definition of Variable Sampling along with its overview.


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