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I would like to say that zero defect is achievable but it depends on how we are doing its interpretation. Theoretically and practically it is not possible to achieve zero defect. But I would say zero defect means get things right at first time. So we can achieve zero defect. For example if I talk about to finish the project. During finishing of project we have so many steps which are having their own deadlines and conditions. If we are completing all steps in time with all requirements and finish our project on time we can say that there was no defect because whatever has been planned earlier is achieved now. Also lean manufacturing and continous improvement focus on preventive maintainance to reduce and eliminate waste activities so that we can achieve our targets.