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Developing Project Schedule is an important activity during planning of project development. Originally, project scheduling and planning were developed purely based on timing and sequence of project activities, assigning tasks and indicating due dates for the completion. This approach known as a deterministic schedule, assumes fixed start and end dates, as well as predetermined durations for all tasks.

Along with project complexity, deterministic schedule is less precise to reflect the actual situation during execution. The current approach will include uncertain event during project, which may have impact to the project. This is called as probabilistic schedule, which the duration is not a fixed and determined from the probability distribution. A key strength of probabilistic scheduling is its proficiency in dealing with uncertainty.

Project Schedule Risk Analysis (PSRA) is a method to improve forecasting accuracy and effectiveness by considering uncertainty. In general, PSRA has three steps: (1) create critical path, (2) estimate uncertainty durations, and (3) perform risk analysis.

Figure 1 shows probabilistic distribution of the project completion or on-stream date. The P50 on-stream date (50% probabilistic to occur) is commonly used as a basis for project economic calculation. The left figure (pre-mitigated) shows the probabilistic where there is no mitigation applied to the associated risk identified, where right figure (post-mitigated) shows where risk mitigation is applied. It is shown that P50 project on-stream date shift to left (comes earlier) when mitigation plans are applied.

Medco E&P Indonesia has implemented PSRA as a standard. It was proven that the P50 project on-stream schedule is closer to the actual compared to deterministic schedule. This approach has become a robust tool to support management decision process and reference to deliver the project.