Quality of a software product cannot be improved by testing it again and finding and fixing the defects. It needs to be built in the product while development using good processes and methods. Any amount of testing cannot certify that a product is defect-free. Good procedures can make good software. No software can be considered as defect-free even if no defect is found in the test interation defined for it. We can only say that no defect has been discovered till that point of time using those many test cases.