不大懂这两个名词，请大家解释一下

Estimate Purpose and Accuracy Are Not Clarified
Not all estimates are created equal. Not all estimates are produced for the same reasons or have the same precision. Anyone who produces estimates of time or cost without clearly stating the intended use and accuracy is contributing to this problem and is asking for trouble later in the project.
Estimates produced for different reasons, at different points in the project life cycle, have different degrees of accuracy. This is primarily because there is more uncertainty at the start of the effort, which then tends to diminish as the team develops a greater understanding of the requirements, approach, and technical feasibility. Once a detailed work breakdown structure is developed, bottom-up estimating that can then be performed produces considerably more reliable estimates. As an example of this, the initial project quote may be $16 million and 20 person-years of effort. However, that could just be a rough order of magnitude estimate whose accuracy might be -25 to +75 percent, which was developed to explore system affordability. By the time the lowest level activities are estimated for the purpose of predicting effort and cost at a detailed level, they could be accurate from -5 to +10 percent. Also by the time the lowest level activities are estimated, the overall project might have become $24 million dollars and 30 person-years of effort, which is well within the initial bounds. However, unless the uncertainty in the original estimates is stated up front, there could be misunderstandings, disappointment, and difficult discussions as the project progresses. This is why good estimating practices and clear communication are essential right from the beginning. However, what if a reduced uncertainty level is required at the beginning? A common technique for dealing with this situation is the "rolling wave" or "moving window" approach. The next project phase (or two) is estimated with increased accuracy, and later phases receive the order-of-magnitude values. As the project progresses, the "window of accuracy" also expands, until eventually it includes the entire project. Two key points must be made with respect to this technique. First, the accuracy of even the initial phases cannot be increased without doing the requisite work to resolve uncertainties and define all tasks to the lowest level. Also, good communication again becomes critical because it may not be readily apparent to senior management or the clients that half of the estimates are at the plus-or-minus 15 percent level, while the remainder are at the plus-or-minus 50 percent level. As before, estimation purpose and accuracy should be clearly stated.