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Defining Activity Risk

To use the risk analysis feature in Open Plan, you must enter duration probability information for at least one activity in a project. More typically, you enter probable durations for all activities that involve at least some degree of uncertainty in their planning.

Note: You can enter probability information for any type of activity that does not use a calculated duration (for example, subprojects, external subprojects, or hammocks).

To enter risk information at the activity level, use the Risk tab of the Activity Details dialog box.

For each activity on which you want to model risk, you can enter an optimistic (minimum estimated) duration and a pessimistic (maximum estimated) duration in addition to an original duration. You can also enter a weight factor (0-100) for the Probability of Occurrence that indicates the probability that the activity will exist at all. For example, assume you have the three activities in the following table.

Activity Probability of Occurrence
A: Write software 100
B: Test software 100
C: Fix software 20

Activities A and B will always happen, but we might judge that there is only a 20% chance that following testing, we have to go back and fix the software. All regular Open Plan features ignore this field (and always consider all tasks) except Risk Analysis, which would consider activity C only 20% of the time. If Risk Analysis chooses to ignore the task on any given iteration, then any successors of C are deemed to follow immediately on from B (as if C had zero duration).

You must also enter one of the following distribution shapes to define how Open Plan will sample the range of possible durations:

  • Beta: Probable outcomes cluster around the mode value of the distribution. Outcomes at the extreme values of the distribution are very unlikely.
  • Normal: Probable outcomes cluster symmetrically around the center of the distribution.
  • Triangular: Probable outcomes cluster around the mode (most likely) value of the distribution. Outcomes close to optimistic and pessimistic values of the distribution still possess a significant probability.
  • Uniform: Each outcome is equally likely.
Attention: For a complete description of risk analysis calculations in Open Plan, see "Risk Analysis Calculations" in the Deltek Open Plan Developer's Guide.

Note that Open Plan requires a mode or “most likely” duration for the triangular and beta distribution shapes. In these cases, the original duration of the activity is assumed to be the mode duration. For symmetrical distribution shapes (uniform and normal), a mode duration is not required for risk analysis processing. However, because virtually all processing by Open Plan other than risk analysis uses the original duration field, you should always enter this duration, no matter what type of distribution shape is specified for the activity. In fact, Open Plan checks for the validity of the duration field by verifying that its value falls between the optimistic and pessimistic duration before you can store the information for the activity. An activity with a blank duration field is interpreted by Open Plan as having zero duration.

The following examples indicate how to specify duration probabilities for activities.

Example 1: You estimate that an activity can have a duration of 3, 4, or 5 days. Each duration is equally likely.

  • Optimistic = 3d
  • Pessimistic = 5d
  • Duration = 4d (not used during risk analysis)
  • Shape = Uniform

Example 2: You estimate that an activity could have a duration anywhere between 10 and 30 days based on a normal probability distribution.

  • Optimistic = 10d
  • Pessimistic = 30d
  • Duration = 20d (not used during risk analysis)
  • Shape = Normal

Example 3: You estimate that an activity could have a duration anywhere between 10 and 30 days. You predict the most likely duration to be 12 days.

  • Optimistic = 10d
  • Pessimistic = 30d
  • Duration = 12d
  • Shape = Triangular

Example 4: You know that an activity has a duration of 2 days.

  • Optimistic = blank
  • Pessimistic = blank
  • Duration = 2d
  • Shape = blank

Open Plan also enables you to designate one or more activities in a project as “key activities.” For key activities, Open Plan stores the most detailed risk analysis information possible. You can then use a risk histogram view to display detailed histograms for the probability distributions for such information as the early dates of selected milestone activities.

Note: The designation of a large number of key activities can result in the creation of an unmanageably large Risk table when risk analysis is performed. Therefore, it is recommended that the number of activities flagged as key activities never exceed a dozen or so per project.

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