这是因为我所做的自底向上的详细估算可能会出错。
This is because my detailed, bottom-up estimates are probably wrong.
当详细设计进行到一定阶段，最根本的详细设计已经完成时，就要基于项目的明确范围进行详细估算。
When the detailed design has progressed to a point when the essential details are known, a detailed estimate is made on the basis of the well defined scope of the project.
详细估算或者称为明确估价是在工程范围基本明确、详细设计正在进行、项目的基本特征已经确定时做出的。
The detailed estimate or definitive estimate is made when the scope of work is clearly defined and the detailed design is in progress so that the essential features of the facility are identifiable.
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