What is the difference between Sampling plan and  Inspection, my knowledge Sampling is used to accept or reject a product lot however the Inspection is used to judge the quality of the product. but still i need more clarification.

 

Hi Ahmad,

Sampling plan gives howmany pieces/items you have to inspect out of received lot.

satya

Hi Ahmad,

Inspection is the process of verification to know whether the thing being inspected is meeting specified parameters/requirements. Sampling is just a statistical method to select a suitable sample size to inspect the lot.

As practically it is not possible to carry out 100% inspection on account of cost involved, long time , destructive nature of some tests sampling technique is used. Sampling uses samples (some quantity form population) and using this decision is taken about correctness of the entire population. Sampling is based on statistical theory and if randomness is ensured in picking samples, it can give correct estimate about entire population. Based on the levels of risks (bad thing being accepted as good) that is acceptable there are different sampling plans with different AQLs (acceptable Quality limit) and accompanying accept reject no for specified sample size. . Every organisation is required to choose based on the nature of product/service. eg. If it is consumer elctronics like TV manufacturing AQL of 0.65 is used for components incoming insepction. If it is office automation equipment AQL of 0.4 may be used. Again it may vary from component to component.

So sampling is not an alternate for inspection. Even when we use sampling what we do is inspection

Thanks Madhavi for your input and beneficial information.

Cheers Madhavi! Really helpful

I agree with Madhavi. Sampling and inspection need not be confused together. Inspection is checking whether the part conforms with the spec or not. Sampling is a methodology to decide the status of the entire lot. Based on the sample size you are going to decide whether the lot is acceptable or not. After taking samples you are going to inspect it obviously.

Sampling plans when adopted for inspection help to reduce the cost of inspection. But the sampling plan must be chosen with consideration to the operating characteristics of the sampling plan. These are plotted based upon the statistical calculations.

It is generally considered a good plan if for a given criteria of acceptance , say batches with 0.25 % defectives or less are acceptable, then a plan would be expected to be a good one if out of 100 such batches at least 95 are found acceptable by adopting this plan. You may refer to the indian std IS- 2500 Parts 1,2 published by the Bureau of Indian stds, new delhi, which deals in some detail about the use of sampling plans for attribute inspection.It also guides you to the selection of a plan.

The problem with the plans is that a few batches with good quality may get rejected and similarly a few no of batches with more defective can get accepted. This being called the risk to the producer ( rej good batch) and for the consumer ( acc bad batch). This risk has also been calculated for the plans based upon the assumption of a random sample and probability of occurrence in the drawn sample.

sanjai

Most of the time tolerance level ( acceptable level of defectives) for a type of defect is pre-decided in a product . This helps to set the AQL (acceptable quality level) for this defect in the batch of the product.

The AQL level depends on the impact the defect shall have upon the performace , life, reliabilty, serviceablility, safety, etc.

Generally critical defects rendering a product unsafe for use are not permitted.

Those having very little or no effect on performance are treated as minor such as Color /shade of paint. 

This can be also derived form the QFD analysis or the FMEA analysis. Of course the customer is always right and even cosmetics/asthetic looks can become important.

 

Another pertinent assumption is that the batches to be inspected have some percentage of defectives, those with more defective  are to be identified and segragated by the sampling process.

 

sanjai

 

Hi Ahmad

Just to add on to very good explaination given by Sanjay, you can also refer BIS SP 28:1994 handbook on statistical quality control. It gives coplete information on operating curves and sampling.

Thanks Madhavi - for adding a useful resouce-the Handbook by BIS, just to add further for those who are making products for use in the defense forces the applicable standards which are equivalent to IS 2500 are DEF131A ( british std) and MIL-STD-105E ( earlier 105D  US std). The defense sector  use Normal inspection Level-II , either single or double sampling plans. Special plans with reduced sample sizes are used for destructive tests.

As we know most of the quality system requirements like the QMS have their origin in defense stds and similar high end space/avionics/military application of products & systems. The stds and sampling procedures were already in place in such applications many many years ago and have later came to the civil applications.

An online calculator of the Accept no & Rej no is available from a link in Wikipedia search for the MIL Std 105E on the page SQC online, if we know the batch size and  AQL % for the defect using a certain level of inspection i.e Normal level -II or ant other the Acc & Rej nos are calculated and displayed.This may be useful to those who do not have the tables readily available.

regrads : sanjai

Thanks so much Madhaviand Sanjai for your information.

Hi Madhavi

Could you please provide the author name? I would like to buy this book, I am not finding enought information in Google.

Thanks

Madhan

Hi Madhan,

This book is from BIS(Bureau of Indian standards). You can check with any of BIS sales office.

Madhavi

 

info given in this topic is very useful,

In garments manufacturing AQL is playing important role

tks to madhavi

Babu.A

Thanx 2 all for view on Sampling plan vs. inspection.

Umesh Dimri

ITC Ltd. Haridwar

hello,

  Thanks for your useful information about level of rejection identification.

Hello,

Can some one brief about AQL (Acceptable Quality Limit). I didn't get it exactly.

Shrikant,

Sanjay W has given one example to explain AQL as follows It is generally considered a good plan if for a given criteria of acceptance , say batches with 0.25 % defectives or less are acceptable, then a plan would be expected to be a good one if out of 100 such batches at least 95 are found acceptable by adopting this plan.

With the help of AQL, you can decide how many defects can be acceptable with the help of threshold %age.

Today time is zero defect , and in final inspection we are giving some acceptance of rejection means we are sending some rejection to customer. is this is ok.

Ramesh Rangera

@Ramesh,

This acceptance level is from customer’s as well as inspection stand point; how much tolerance can be given. If you want to go ahead with zero defect with 100% confidence level then you can go ahead with 100% inspection instead of sampling the data for inspection. Sampling method is used to control the inspection cost, else if will be higher.

Cheers!

What is IS2000 standard for sampling plan , where this available . is it perfect std for sampling.

Sampling plan is one of the methods used in Inspection. The choice of sampling plan depends upon the volume of rejections occurring.

Hello All ,

If we check the posted question carefully it was difference between Sampling Plan & Inspection , to my knowledge sampling plan is the system we make to inspect the products / services as per defined & agreed criteria between customer & Supplier that is termed Acceptable Quality Level AQL . Numerous STD's are available from MIL ~ IS ~ BIS for sampling plan , irrespective of the sampling STD's we can formulate the sampling plan on the basis of the Quality & risks involved such as producers (alpha) risk and consumers (Beta) risk . Operating Curve just shows the realtionship Lots defectives per million in X axis along with probability of acceptance of that particular lot at Y axis

Whereas Inspection is the verification part of the sampling plan we used in order to document the activities we perform undr sampling plan.

In short Inspection is the nessacery activity we do under guidance of sampling plan to identify the nature of the so called population .

Hope it clarifies ( Correct me if wrong)

Regards

Mahi

Dear All

As defined by Mr.Bhandari it is right and also I wanted to add some thing to elaborate it.

Inspection is the process which is done to find out the kind and number of deviations occurring to the product(s) comparing with respective to the master sample or with the specified specifications. Some times inspection is also done to rectify the deviation which is done on line or off line.

Where as sampling plan generally carried out after the process or offline whether to accept or reject the lot comparing with the defined limits.

Mr Ramesh rised good point on acceptance level. Customer wants zero defect products. Selecting sample plan based on AQL means, supplier or inspector who is accepting the material based on sample plan is 95% confidence the accepted lot of material have rejected material which is lessthan the % of AQL. It means the accepted material is not zero defect. The only one way to confirm the zero defect is 100% inspection.

But it is not possible practically to inspect 100% in all products / components at final inspection of supplier or inward inspection of customer.

So, Customer and supplier should have common agrement on AQL level. Marketing people may be not accepts this but fact.