Bottom-up estimates are used when project baselines, or a cost- control type of estimate, are required. The approach is called "bottom-up" because estimates of this type begin with estimating detailed costs of the project and summarizing these at their appropriate level. The work-breakdown schedule (WBS) can be used for this "roll-up." This kind of estimate is that it produces accurate results. The degree of that accuracy is mainly a function of the level of detail taken into account: as more and more detail is added, the calculations converge statistically towards the most likely cost estimate, or cost range estimate,. Of course: the cost itself of the time required to complete such a relatively detailed estimate is higher, as is the amount of time needed to produce the estimate.