Estimating the duration uncertainty and probability of tasks

Before carrying out risk analysis on a project for the first time, you should estimate the duration uncertainty and probability of the tasks in the project.

The duration uncertainty of tasks provides Risk Analysis with the minimum and maximum duration range between which a random duration should be selected for each task for each iteration. For example, if a task has a duration of 10 days, but you feel that the task could take between 8 and 12 days to complete, depending on circumstances, you could specify a minimum value of 8 days and a maximum value of 12 days in terms of duration, or a minimum value of 80% and a maximum value of 120% in terms of percentages.

The probability of tasks provides Risk Analysis with the likelihood, in percentage terms, of a task taking place. Specifying a task probability other than 100 makes a task a "risk event" - ie a task that has a probability of occurring and that will not definitely occur. For example, you may want to create a "risk event" task to represent a possible delay in the delivery of certain items. If you specify task probabilities, the probability of risk event tasks is taken into account in each iteration of risk analysis, meaning that the risk event tasks will be included in some iterations but not in others. If you specify that a task has an 80% probability of taking place, the task will appear in - and therefore affect the duration of - roughly 80% of risk analysis iterations.

Estimating the duration uncertainty of tasks is a necessary part of risk analysis, but you can choose whether or not to estimate the probability of tasks - and you can estimate the probability of as many, or as few, tasks as you wish. If you do not estimate the probability of a task, it is assumed to be 100% likely to occur.

You can estimate the duration uncertainty and probability of tasks in the following ways:

  • On individual tasks, by entering precise minimum and maximum durations and a probability percentage into fields for each individual task.
  • Using code libraries, by creating codes that define the minimum and maximum duration values, and the task probability, all in terms of percentages. You can then assign the codes to tasks.
  • For duration uncertainty only, using default percentages, by entering minimum and maximum values in terms of percentages in the Risk Analysis dialog. It is not possible to use a default percentage to estimate the probability of tasks.

You can use whichever method suits you, or you can use any combination of the methods. For example, you can estimate duration uncertainty by entering minimum and maximum values in terms of duration on a number of individual tasks, then apply default percentages to the rest of the tasks in the project.

If you use a combination of methods of estimating duration uncertainty and task probability in a project, Risk Analysis determines the uncertainty of each task's duration and the probability of the task as follows:

  1. If the task has minimum and maximum duration values and a task probability entered into fields, these are used.
  2. If the task does not have minimum and maximum duration values and a task probability entered into fields, but has been assigned a code from a risk analysis code library, the values that have been entered against the code are used.
  3. If the task does not have minimum and maximum duration values and a task probability entered into fields and has not been assigned a code from a risk analysis code library, but the bar or a parent task has been assigned a code from a risk analysis code library, the values that have been entered against the code are used.
  4. If the task does not have minimum and maximum duration values and a task probability entered into fields and the task, its bar or a parent task have not been assigned a code from a risk analysis code library, but default minimum and maximum duration percentages have been entered on the Risk Settings tab of the Risk Analysis dialog, these are used; the task is assumed to have a probability of 100%.
  5. If the task does not have minimum and maximum duration values and a task probability entered into fields, the task, its bar or a parent task have not been assigned a code from a risk analysis code library and default minimum and maximum duration percentages have not been entered on the Risk Settings tab of the Risk Analysis dialog, the task is not affected by the risk analysis procedure.

This is illustrated by the following flowchart:

A flowchart illustrating how to estimate duration uncertainty

100% complete tasks are excluded from risk analysis: it makes no sense to include them, and their durations cannot be altered.

Partially-completed tasks are included in risk analysis: new durations are generated for the uncompleted sections of part-completed tasks in each iteration in the same way as they are generated for tasks with no progress.

When estimating the duration uncertainty of tasks, if the minimum duration that you specify would result in a task being given a duration that is less than the duration of its completed section during an iteration, Risk Analysis does not reduce the duration of the task to make it 100% complete. If the maximum duration that you specify is less than the duration of the completed section of a task, Risk Analysis excludes the task from the process and warns you.

Once you have estimated the duration uncertainty and probability of tasks in a project, you specify the method(s) you have used to do so on the Risk Settings tab of the Risk Analysis dialog.

Related Topics:

Estimating the duration uncertainty and probability of individual tasks

Estimating the duration uncertainty and probability of tasks using code libraries

Estimating duration uncertainty using default percentages

Specifying how you have estimated the duration uncertainty and probability of tasks