• Duration Uncertainty: This is a broad classification that refers to any one or more of a multitude of factors which can influence the working time taken to complete a package of work. Examples of duration uncertainty factors include:
◊ Quantity uncertainty: Uncertainty over how many units of work are required to be completed.
◊ Rate / productivity uncertainty: Uncertainty over how many hours are required per unit of quantity.
◊ Staffing uncertainty: Uncertainty over how many workers will be available to complete the specified works.
• Risk Events: These are events that may or may not occur, but if they do, will impact on one or more aspects of project timing. Examples of different types of risk events are:
◊ Engineering / Design Risks: eg.
◊ Procurement / Fabrication / Supply Risks: eg.
◊ Construction Risks: eg.
◊ Commissioning Risks: eg.
◊ Business Risks: eg. “There is a risk that permit conditions may be changed due to a change of government, resulting in a reduced requirement for environmental rectification and associated schedule savings.”
• Logic Uncertainty: In basic planning, we’re limited to one type of logical link between activities. They’re either always there, or we don’t put them in at all. However, in probabilistic scheduling, we can model links that may or may not exist between tasks, or even choose between alternate pathways for the schedule to follow…
◊ Probabilistic Links: These are links that may or may not exist between two tasks. The link is assigned a probability of existence and switches on an off accordingly during simulation, similar to a risk event. Probabilistic links could be used to model, for example, the situation in which there’s only a 70% chance that a permit may be required to commence a particular construction activity.
◊ Probabilistic Branching: This can be used to model situations in which there are two or more potential and mutually exclusive solutions or pathways for completing an objective. An example of this might be building a bridge versus building a tunnel to achieve grade separation from a busy road. Each pathway is assigned a probability (the sum of which must total 100%), then either is selected in each iteration when the project is simulated, according to their percentage probability of selection.
• Calendar Uncertainty: Both duration uncertainties and calendar uncertainties affect the overall duration of a task. However, unlike duration uncertainties which influence the working time taken to perform a package of work, calendar uncertainties determine the times in which this work can be performed. There are two main types of calendar uncertainties:
◊ Weather Uncertainty: Weather uncertainty refers to both the variations in normal working downtime associated with inclement weather, as well as downtime associated with one-off weather events such as cyclones (elsewhere typhoons or hurricanes).
◊ Working Roster Uncertainty: At the early stages of a project, there may still be uncertainty surrounding the roster arrangements for workers. Will the office staff work a 7½ hour day or an 8 hour day? What shift rotation or time on / time off patterns will the construction staff work? These types of uncertainties can have significant impact on the project completion date.