If the work is represented by a single activity with 3 estimations (80d, 100d, 125d) result of Monte Carlo simulation performed with Spider Project tool would be as following: • 100d = 43.9% • P75 = 108d Different risk management software programs, however, use their proprietary levelling algorithms and create different resource-constrained schedules for the same projects. This is due to the absence of a mathematical solution for finding the best (shortest) schedule when project resources are constrained. The risk analysis in any external risk simulation tool is likely to be not reliable when the project has limited resources. Distribution calculated by an external tool assumed that project will use resource assignments as the external tool recommend it. Activity lags & dependency uncertainties A project may have uncertainties not just within activities but also between them. Uncertainties could be in project dependencies, external supply dates and dependency lags. Dependencies Figure 9 - Distribution based on a single 100d activity. If the same work is presented as ten consequence activities. (8d, 10d, 12.5d), the result of the simulation will be different: • 100d = 27.3% • P75 = 103.7d Some work may have uncertainty in the type of dependency when it is unclear which type of dependency has to be applied: ‘Start-to-Start’ or ‘Finish-to-Start’. This type of uncertainty may be represented by probabilistic branching but not many scheduling tools have this feature yet. Figure 11 - Dependency uncertainty. External dependency Figure 10 - Distribution based on ten 10d activities. It does not look logical. Probability distribution should not depend on the way the work is presented in the project schedule. Activity durations depend on the productivity of assigned resources: the higher the productivity, the faster all ten activities will be performed. Therefore, the durations of all ten activities are strongly correlated with each other. If the source of uncertainty (resource productivity) was simulated instead of the result (activity duration) this problem would not exist. The majority of risk analysis tools don’t allow to work with the true types of uncertainties. Resource uncertainties Uncertainties in resource availability may impact activity durations, sequence of the activities and even project critical path. It means that in each iteration, resources must be levelled. Figure 12 - Activity Lag. The date of the external deliverable may be uncertain. An external dependency is usually represented as a milestone but milestones can’t have 3 durations (it is always zero). If possible, it is recommended to represent external dependency as an activity, instead of a milestone. Lags Lag is the delay of a successor activity and represents time (or volume of work) that must pass before the second activity can begin (or finish). Many risk simulation tools can’t simulate lag uncertainties. As a workaround lag (lead) could be replaced with ‘lag activity’ and 'lead activity'. 00:0 8
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