Assigning uncertainty to the amount of work in the project | Vose Software

Assigning uncertainty to the amount of work in the project

The uncertainty in the amount of time it takes to complete a task is dependent on three things:

    1. Scope uncertainty: the uncertainty in the actual amount of work that needs to be done;

    2. Productivity uncertainty: the uncertainty in the speed with which the work can be executed;

    3. Risks: any risk events that interrupt the execution of that work.

Tamara deals with these three aspects separately. This approach makes it far easier to obtain realistic estimates of total schedule uncertainty. We begin here by considering the uncertainty in the actual amount of work that needs to be done. Steps 3 and 4 deal with productivity uncertainty and risk events. Describing work amount uncertainty is composed of two parts:

    1. Assigning quantitative definitions to descriptions of levels of work amount uncertainty;

    2. Applying these levels to tasks within the project

a.     Assigning quantitative definitions

In the Task Overview tab, click the Work Amount Uncertainty icon:

This opens the following dialog:

The nine descriptions combine whether the original estimate of scope is biased (Conservative, Neutral, Optimistic) with the amount of uncertainty (Small, Medium, Large). These values represent the range of adjustments that will be made to the imported task duration values during a simulation. For example, the list above shows ‘Conservative large’ to have {Minimum, Most Likely, Maximum} values of (-30%,-5%,10%). If applied to a task with a duration of 20 days, Tamara interprets this as meaning that the duration could actually be between 14 days (i.e. -30%) and 22 days (i.e. +10%), and most likely the cost would be about 19 days (i.e. -5%).

The definitions can be altered in this table by simply clicking the relevant cells in the table, and then clicking Save. Clicking the Revert to Default Values button will reset the definitions to those set in the Application Settings tab.

The {Minimum, Most Likely, Maximum} values are interpreted internally as inputs to a Modified PERT distribution with shape factor = 3. This provides a natural-shaped distribution with a good level of weight to values near the extremes. For example, {-10%, 0%, 20%) would be interpreted as follows:

b.    Applying work amount uncertainty to tasks

Project planners who have created the imported Primavera or Project schedule will have assigned a duration for each task. You can now specify a level of uncertainty around these durations.

This is achieved in Tamara by selecting the phrase that most reflects the uncertainty around the fixed value in the Work Amount Uncertaintyfield in the Main View. For example, in the following illustration, task 57 is being assigned a ‘Conservative large’ uncertainty, meaning that assuming that the rate of work is usual, the 8 days planned for this task is likely to be on the large side. The % variations around the base value that will be applied are shown next to the definition:

For tasks with no scope uncertainty, leave the field empty (or select the blank option from the drop-down list).

For some tasks, none of the definitions may reflect the uncertainty in scope. This usually occurs when there is a task for which the uncertainty distribution has a particularly long right tail. In such situations, select Custom.  This will open the following dialog box:

You can now enter a distribution specific to that task, either in days or in a percentage variation around the base estimate of duration. The cell will show these three value for easy reference:

You can edit the entered values by selecting the cell again, and then selecting Custom.

You can quickly assign scope uncertainty to a set of tasks, which is very useful if you have a large schedule. In the default view, where tasks are shown at various levels (parent and child), selecting a parent task and then picking an uncertainty description will assign this scope uncertainty to all child tasks below it. For large schedules, you may find that the easiest approach is to first define the work amount uncertainty at Level 1 or 2, and then edit for individual sub-tasks where there are exceptions.  

For the simplest model, this is all the information required to be able to run a simulation and review the uncertainty of the schedule. If you do not need to incorporate productivity risk factors or risk events that would also impact the schedule, you can proceed to Run a simulation and review the results.