There are lots of articles researching on pull system model, which often neglecting the defect of parts or assuming the defectlevel equals to zero directly in production process.
关于拉式系统模型的研究文献很多，但它们往往忽略生产过程零部件缺陷的存在或者直接假设缺陷水平为零。
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Figure 13: By graphing "Severity" of defect against "time" to close, we see that some Level 1 defects have taken time to be resolved.
Fourth, there are no defect attributes that describe users' feelings, so developers can never know the customer satisfaction level until after release.