What is quality in a project? If we have a project that requires manufacture 10000 cardboards boxes for a luxury fashion house. How can we measure the quality in this project? Someone will tell us that the boxes should be resistant, others will request that the boxes should be provide with a cool design and others that the boxes must could carrier more than 5 kilograms. But if our client only needs a simple cardboard box to storage apparels and the boxes will not be used to shipping why are we requesting these extra features?
The PMBOK defines the quality project as degree to which the project fulfils requirements. In this case a simple cardboard box with the requested measures will provide the enough quality to the project.
The meet the expected quality the Project needs a quality control or test phase to ensure that there are not errors in the final product.
How we can test the final product? It depends on the product. In our boxes project we have to ensure that the boxes have the expected measures. But have we to measure 10000 cardboards boxes?
When we prepare a quality control or quality assurance process we have to apply a cost benefit rule. So the cost of quality test should be lower than the cost of reprocess or non-conformance. May be in our project measure 10000 boxes could be more expensive that re-manufacture 100 boxes.
To avoid extra cost in the quality there are statistical techniques that allow test a sample of boxes and ensure a certain level of quality.
If the cost of measure one box is one dollar, and we have a statistical method that ensures that if we take a sample of 2000 units the confidence level is 5%. This means that 95% boxes should have the correct measures. So if we measure 2000 units we can ensure that we only have 500 boxes with incorrect measurements that we will reprocess.
In other hand we can measure 10000 units to ensure that we have 0 incorrect boxes
As we can see above the quality test with a sample and using statistical methods have better cost benefits ratio.