a deliverable-oriented WBS. Work breakdown structures that subdivide work by project phases (e.g. preliminary design phase, critical design phase) must ensure that phases are clearly separated by a deliverable also used in defining entry and exit criteria (e.g., an approved preliminary or critical design review). Level of detail One must decide when to stop dividing work into smaller elements. For most projects, a hierarchy of two to four levels will suffice. This will assist in determining the duration of activities necessary to produce a deliverable defined by the WBS. There are several heuristics or "rules of thumb" used when determining the appropriate duration of an activity or group of activities necessary to produce a specific deliverable defined by the WBS. The first is the "80-hour rule" which means that no single activity or group of activities at the lowest level of detail of the WBS to produce a single deliverable should be more than 80 hours of effort. The second rule of thumb is that no activity or group of activities at the lowest level of detail of the WBS should be longer than a single reporting period. Thus if the project team is reporting progress monthly, then no single activity or series of activities should be longer than one month long. The last heuristic is the "if it makes sense" rule. Applying this rule of thumb, one can apply "common sense" when creating the duration of a single activity or group of activities necessary to produce a deliverable defined by the WBS. A work package at the activity level is a task that: can be realistically and confidently estimated; makes no sense practically to break down any further; can be completed in accordance with one of the heuristics defined above; produces a deliverable which is measurable; and forms a unique package of work that can be outsourced or contracted out. Coding scheme It is common for work breakdown structure elements to be numbered sequentially to reveal the hierarchical structure. The purpose of the numbering is to provide a consistent approach to identifying and managing the WBS across like systems regardless of vendor or service. For example, 1.1.2 Propulsion (in the example below) identifies this item as a Level 3 WBS element, since there are three numbers separated by two decimal points. A coding scheme also helps WBS elements to be recognized in any written context and allows for mapping