HomeTroubleshooting@RISK with Projects"Can't graph ... single calculated value" (@RISK 4.x)

6.28. "Can't graph ... single calculated value" (@RISK 4.x)

Applies to:
@RISK 4.x For Project

Problem:
After running a simulation in @RISK for Project, all the statistics and graphs are reporting that the output had no variability. The error, "Can't Graph... Single calculated value", also occurs when drawing the histogram. There are @RISK input distributions assigned to the output's predecessor tasks. Thus, the input functions should be causing variability in the output being analyzed.

Response:
Typically this problem occurs when constraints have been added to tasks in such a way that they prevent the output in question from varying.

With Palisade's @RISK for Project, we recommend that all task constraints be set to "As soon as possible" for forward scheduling projects and "As late as possible" for backward scheduling projects. Task constraints such as "Start no earlier than", "Finish no later than", "Must start on", or "Must finish on", may have a significant impact on Monte Carlo simulations and may result in single value output errors. Constraints are always honored when @RISK performs random recalculations during the simulation and the use of constraints that fix the start or finish dates must always be carefully considered before adding them to the schedule network.

Different constraints will have varying impacts on a project. For example, "Start no earlier than" cuts off early task completions and will make a project schedule longer. "Finish no later than" cuts off late finishes and will make a project schedule shorter. A project plan with multiple "Start no earlier than" and "Finish no later than" constraints will likely be constraint bound and might need to be modified for Monte Carlo simulation.

For more on using constraints in Microsoft Project, we recommend Dynamic Scheduling with Microsoft Office Project by Eric Uyttewaal.

last edited: 2005-07-01

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