HomeTechniques and Tips@RISK: General QuestionsPrecedent Checking (Smart Sensitivity Analysis)

2.8. Precedent Checking (Smart Sensitivity Analysis)

Applies to: @RISK for Excel version 5.x–7.x

When I run my model in @RISK, it seems to take a long time before the first iteration. The status bar shows that it is checking precedents. Is something wrong?

Precedent checking (also known as precedent tracing or Smart Sensitivity Analysis) is a new feature in 5.0 and later versions. Its purpose is to prevent @RISK inputs from incorrectly showing up in Regression/Sensitivity analysis such as tornado graphs.

For example, consider a simple model with two inputs. The two inputs are correlated – let's say to the full extent, with a correlation coefficient of 1.0. One input is used in a calculation for a RiskOutput. The other input is not involved in any calculation which impacts the RiskOutput. In earlier versions, both inputs would be displayed as having equal impact on the output. With precedent checking, @RISK determines that only one of these inputs contributes, and filters out the other one from graphs, reports, etc.

The tradeoff is that it may take quite a bit of time to go through the full precedent tree before a simulation is run. By turning off data collection for some or all inputs, that process can be sped up, though at the cost of not being able to analyze those inputs.

Does Smart Sensitivity Analysis have any limitations?

Certain formulas are correct Excel formulas, but Smart Sensitivity Analysis cannot work with them. Either their values can change at run time in ways that @RISK can't predict, or they are too complex and would take to much time when the simulation starts. These include:

In all the above cases, calculations are still done correctly during your simulation; it's just that for these cases @RISK cannot trace precedents.

If your model contains one of these formulas, when you start simulation a message will pop up: "could not be parsed", "invalid formula", or similar. To proceed with the simulation, click the Yes button in the message. If you want to prevent this message from appearing in the future, either change the formula (if you can), or disable Smart Sensitivity Analysis for this model. To disable Smart Sensitivity Analysis, click the Simulation Settings icon, select the Sampling tab, and change Smart Sensitivity Analysis to Disabled. Click OK and save the workbook.

Last edited: 2016-04-26

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