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inner relationships between each tasks and milestones. This step is always done by WBS. The second step is to construct a network diagram. It has two major methods, Activity on Node method (AON) and Activity on Arrow method (AOA). AON method use nodes to represent tasks and arrows represent precedence relationships only. AOA method use arrows to represent tasks and the position of arrows represent precedence relationship. The next step is to estimate the time coefficients of each task. The time coefficients mainly include ES (earliest start time www.51lunwen.org/projectmanagement/ ), EF (earliest finish time), LS (latest start time), LF (latest finish time), FF (free float) and TF (total float). The forth step is to determine the critical path. Commonly those tasks with zero free float are in the critical path. The last step is to update as project progresses. As it is stated before, the expected value of duration will be replaced by the real one in project management practice, so the PERT chart should be modified to cope with the new situation.

The benefits brought by PERT is obvious:
1) PERT promotes identification of the critical path and makes this visible for managers.
2) PERT promotes identification of late start, early start, and slack (free float and total float) for each task.
3) a PERT chart exactly defines and makes visible precedence relationships between the tasks identified by WBS.
4) PERT provides for potentially reduced project duration due to better understanding of precedence relationships leading to improved overlapping of tasks where feasible.

And also, there are some limitations exist when using PERT chart:
1) The accuracy of PERT is mainly based on the task identification which is done by WBS and the estimation of task duration. However, there can be potentially hundreds or thousands of activities and individual dependency relationships which can have large influence on the result of PERT chart.
2) The lack of accurate timeframe on most PERT charts makes it harder to show status. And furthermore, some experts said that PERT over emphasized the critical path (Davaadorjin, 2010).
3) Just like Gantt chart, when the PERT charts become unwieldy, they are no longer used to manage the project.

4. Task Identity by WBS
4.1 Decomposition Criteria
The set of decomposition criteria is the first difficult task when using WBS. In order to standardize the business process of WBS in construction area, a lot of organizations and individual researchers put forward their thoughts. The International Organization for Standardization (ISO, 1994) identified eight factors of classification for construction information. These include space (office spaces, recreational, etc.), facility (school, hospital, etc.), work section (masonry, concrete work, etc.), element (floors, stairs, etc.), construction aid (scaffolding, formwork, etc.), construction product (cement, paint, etc.), management (procurement, drawing, etc.) and attributes (size, shape, etc.). These facets were updated in ISO (2001) to include more classes. The ones relevant to this study are construction complexes (industrial complex, transport complex, etc.), construction entities (bridges, buildings, etc.), life cycle phases (production phase, design phase, etc.) and work results (filling, excavation, etc.).

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